1. Falling from a really high point and hitting the ground may cause your partner to freeze.
2. Staying in a aircraft until it explodes may cause your partner to freeze.
3. Any ragdolling caused by anything other than combat (i.e. cars, insurance fraud) has a 50% chance of freezing you or your partner.
These are the three numerous things that froze me and my brother.
The cause? Most likely the transfer of your co-op partner characters voice being transfered to you when you're not even near them. This Is possibly the main cause of system crashes that and ragdolling.
Go THQ test your game, since you obviously didn't test it enough.