Offensive Players on Defense?

The last two games I've played, I had Slim reaper pick me off while i was on Offense, and had Robert Griffin Jr Break up one of my passes.   Why is this happening.

What formation was he in?

Unless it's a new exploit, this is usually the result of not having backups on defense. You're only required to have 3 CBs and 2 safeties on your roster. Formations like dime, quarters, or prevent use more CBs and/or safeties than that.  So to fill out the remaining spots the game will pull from the rest of your players and usually put the highest ovr offense players on the field to fill those spots.