I haven't seen a connection between a bowed wrist and back problems. Playing devil's advocate. Most pros are way more open than amateurs. If the argument that more rotation and side bend caused back problems, then very few amateurs should have back issues since they are nowhere near as open or side bent. That's simply not the case. An injury comes when you go past a specific tissue threshold. This happens for a variety of reasons.
A few reasons you'd pass a tissue threshold are:
1. Overuse (too much volume)
2. End rang of motion at high speed
3. Rapid acceleration or deceleration
4. No warm up - tissue didn't have enough blood flow or hydration and was more likely to tear than stretch
There are probably more, but those are a few I commonly explain to my students. Too much side bend, in theory, approaches two of those issues, but I see many more back problems from a stall/flip with early extension. That usually fits more in the rapid deceleration bucket. Either way, I don't see an overly mechanistic reason a bowed wrist causes back pain. But that's just my opinion.