iPad Wouldn't Rotate Until I closed All Apps

A couple of minutes ago, I realized my iPad was stuck in landscape mode. Rotating wouldn't work and the side switch didn't help. Then I double clicked the home button and closed all apps that were open. The iPad went back to portrait immediately, so I guess it's some app causing this problem. However, I was out of any app when I couldn't rotate. Does that mean there's an app that just blocks screen rotation or is it a software problem?

Uh, and I'll use this thread to ask another question about something weird happening with my iPad, if you don't mind. I was playing Fruit Ninja and used four fingers and repeatedly swiped them left-right. Then, all of a sudden, one of the "blades" transported itself from going left-right to up-down. It kinda inverted. Is this a problem with the touch screen or the app? Thanks.