Why does my game keep minimizing on me during game play?

Then you should start checking what programs do you have that may be executting some fuction causing that problems. I'd start with any anti-virus or firewall, specially if you have a "cracked" software, since maybe they're trying to show a message for "this program is not original" and you have something blocking it, but it still minimizes your game.

Anyway, just check the soft you have running while you play. Try closing most of the unnecessary programs and see if it stops happening.