I was wondering the same thing. I assume your toolpath starts at X0/Y0 and you are using an offset toolpath cutting from the outside to inside, then your z-axis would be dropping. A while back I had a similar issue, cutting a 3d raster toolpath and my cut went deeper with each pass.
https://discuss.inventables.com/t/linear-z-axis-drops-with-each-pass/41608
In my case I ended up replacing the wire for the z-axis and found a overtightened (loose) screw at the terminal block on the carriage. Maybe your issue is similar