Causes of foreign key constraints disappearing

What are some ways foreign key constraints to disappear? I have about 20 databases where some of the constraints have disappeared.

mysql> show create table phppos_sales_items;
+--------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table              | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
+--------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------开发者_开发百科--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| phppos_sales_items | CREATE TABLE `phppos_sales_items` (
  `sale_id` int(10) NOT NULL default '0',
  `item_id` int(10) NOT NULL default '0',
  `description` varchar(30) default NULL,
  `serialnumber` varchar(30) default NULL,
  `line` int(3) NOT NULL default '0',
  `quantity_purchased` double(15,2) NOT NULL default '0.00',
  `item_cost_price` decimal(15,2) NOT NULL,
  `item_unit_price` double(15,2) NOT NULL,
  `discount_percent` int(11) NOT NULL default '0',
  PRIMARY KEY  (`sale_id`,`item_id`,`line`),
  KEY `item_id` (`item_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 | 
+--------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> show create table phppos_sales_items_taxes;
+--------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table                    | Create Table                                                                                                                                                                                                                                                                                                                                    |
+--------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| phppos_sales_items_taxes | CREATE TABLE `phppos_sales_items_taxes` (
  `sale_id` int(10) NOT NULL,
  `item_id` int(10) NOT NULL,
  `line` int(3) NOT NULL default '0',
  `name` varchar(255) NOT NULL,
  `percent` double(15,2) NOT NULL,
  PRIMARY KEY  (`sale_id`,`item_id`,`line`,`name`,`percent`),
  KEY `item_id` (`item_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 | 
+--------------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

This could be a cause. It is described in detail here, though old. Doing alter table after create table could be a cause. Are you doing something similar?