I think I have seen this problem before. My guess is that you sheet out, probably unconciously, when you start thinking about moving to the footstraps.
This causes the board to loose momentum, to sit on its tail and pivot around it, and then the pressure will build in the sail and you catapult.
Remedy: Focus on keeping power in the sail all the time, lean back as you move your feet. You can practice this using a fixed object on lans as a simulator. Hold on to something, lean back, and move your feet without changing your lean (and pull).