If being on the bike is causing your back to go wonky, you'd feel it either while you were in that position, or immediately after trying to straighten back up from that position.

You'd also feel it in similar positions after the initial incident... like bending forward to brush your teeth, or tying your shoes, or reaching down to pet your dog.

Maybe you are too still and steady on the trainer compared to the rollers, or maybe the front and rear of the bike aren't level on the trainer which is throwing you off.

Or maybe you have a kidney stone.

No kidding, they are evil little boogers that can look and feel like back pain, but they will be erratic and unpredictable.

They can have a mechanical event that provokes them, and then not respond mechanically the way a disc would. A good therapist should know within 3 visits whether or not your pain is responding mechanically and bounce you right back to the doc if needed.