Hello,
Microsoft has a "handedness" setting on tablet computers that can trigger this behavior. Because for PCs with the "right-handed" setting, menus will open to the left to avoid blocking the hand.
This article describes this Windows feature in general.
We've never liked that setting and it's one of the first things we change when getting a new tablet PC.
I also found this StackOverflow answer that should allow you to force a setting for your application to avoid this.