You're probably popping a GFCI, either in your breaker box or at the outlet where your lights are plugged into.

I ran into this last year. With 60 extension cords on 3 15A circuits, it's really, really tough to diagnose. This year I used a 4"-5" piece of bicycle tube around each plug/socket held on by a tie-wrap at each end. The only GFCI pop I had this year was due to a permanently mounted flood on the front of the house, which had its drain hole plugged with pine needles from a tree right above it. Had I gone bare, like last year, I'd be going crazy by now with all the recent rain.