Id bet its the drIve after the climb.
i doubt it's my case, as i sleep there in between two consecutive climbing days.
(long drives sometimes give me back issues but rather lumbar than thoracic.)
one could argue it's sleeping in my van, but i am very used to it and never noticed this specific kind of back pain on other van-based climbing trips.
i have a strong feeling it's muscular, as in: something in that area works "more than usual" on this route and reacts by getting stiff overnight.
What i'm a bit lost about, is what exactly gets overworked, is it more a case of under-training the area or bad technique/beta, can i mitigate the issue on site (evening/morning routine) and or prevent it at home (conditioning).