Fumbles are not handled by the AI properly.
If the ball is fumbled, and picked up before the game properly announces the fumble, possession of the ball will remain with the offense even if the defense gains possession. This will result in a safety if the ball is moved into the end zone in this time.
There are many various potential glitches that occur (some pretty awful) due to fumbles.
As shadowhooch said, the only way to prevent forced trades to your team is to man control each team.
If a team is on COM or SKP, it still gets trade attempts. They just aren't shown to you until after they are processed, and just as you can request a trade with a COM team and the com determines if it goes through, they can request trades with you and the com determines if they go through.