As I read on the internet, this issue might be caused by AVG antivirus or similar antivirus programs. Are you possibly using AVG antivirus?
A possible fix to this problem might be playing in windowed mode. If you're unable to change this setting in game you can try to edit the config.wtf file. This file can be found in your wow directory. The following is a quote from the battle.net forums:
Try change to this in config.wtf: SET gxWindow "1"
I play in a resizable window.
Important!: Don't just replace the line in the config.wtf. Either remember or paste the current line in a new notepad document. This way you can always change it back if things get worse.
Let me know if this helped. If not we'll find another way to fix it.
Maddox's post in Client keeps Minimizing itself was marked as the answer