What could be causing my y axis to slip?
(source: all3dp.com)
Your printer is skipping steps in the y-direction. This can have several causes. Take a look into Shifted layer guide on RapRap.org which lists 29 possible problems that can cause this issue and how to fix them.
First items of the list:

Driver current is too low
Driver current is too high
Belt too Loose
Belt too Tight
Loose Set Screw/Grub Screw
Belt or Bearing is binding
Speeds are too high
Acceleration is too high
...

When I was dealing with this issue on my RepRap I had to increase current to the particular driver.