So based off of your description, you're 99% most likely pulling in WAY too much on the bar on relaunch.
If the kite was drifting backwards due to it being too far upwind and flapping before letting go of the kite, you'd probably know to sheet out as it's drifting back to correct.
Since you didn't mention that, my money is on pulling in on the bar and possible a combo of both. Kite too far upwind and luffing and pulling in on the bar too much.
Are you using both hands to relaunch?
Use just one hand and relax your grip and pull back JUST enough to feel line tension at the bar which is your sweet spot for controlling the kite.
Also, move upwind more so there's power in the kite before the assistant let's go.
Hope this helps.