Share 'A new problem : Back crack during finish.?'

I'm wondering if anyone has had a similar occurrence. I'm trying out , on a new build...a WB Lacquer, EM6000. It's been going on fine and I like it so far...but I'm getting up now to the last several coats on the body. It's hanging there in the shop (controlled to 40% on the hygrometer)), and I see as the sun is reflecting on it just so...that there is a 3" line showing on the back near the edge…