I recently discovered that my baseplate makes seesaw movements in Z when travelling back an forth in Y. See video:
This happend during diagonally filling the first layer.
Does someone discovered the same?
Is it possible that the nozzle is too low and pushing down on the center of the bed?
Try another print but when you set the Z level make sure it is way high and see if the bed still lifts. If it does it might be worth emailing support 
Here it is
Either loose mount or loose linear module(s)
if you take the bed off and wiggle the modules you will probably find one if not both have a lot of slop.
support will replace it if it does.
i had the same problem, it got really bad, but it was only one of the two. and i put it there on purpose because it was originally the print head module and i couldnt print anything round.
I think there is a failure in your assembly somewhere. Doublecheck all screws.