ASP.NET Webparts not keeping state after postback

I am working on a portal where the homepage has multiple webparts on the page. Each can be minimized & restored.

The issue here is, once any one of the webparts are minimized and if user clicks on another webpart to minimize that one, the first webpart is now automatically restored. Also, if you navigate to another page and come back, all the webparts ar restored.

How can I hook into the postback for the 'Minimize' event on the webpart? Why isn't the webpart keeping its state?