I suppose you have a 2-part purchase
on each davit: the line is made fast on the davit, goes down through a block connected to the dinghy
, then back up to the davit.
Then, a possible cause is that you coil the line when the dinghy
is hoisted: when you uncoil it, it is twisted on itself. When you pay it out to lower the dinghy, the twists pass through the blocks and this causes the whole system to twist.
I had the same problem on the mainsail
sheet. It was cured the day I stopped coiling the sheet round and began folding it in zigzags.