Partway through a loop, the program stopped because it exceeded a soft limit

I've been working on this subroutine to surface my router table spoilboard (attached). It's a loop that starts at the current origin and works its way up and down Y, stepping over along X by a fraction of the tool width.

The Y moves are relative. The X moves are absolute and controlled by the loop variable. It worked once without issue. I ran it again, and it stopped after a few passes saying the move would exceed (or exceeded?) the machine's negative Y soft limit. How is this possible?