Z Axis Plunging Deeper Than Programmed

[ChubbyRooster]

Ok, new problem similar to the one listed here. Today we were running a tool path (previously run) and something happened where the z-axis plunged as deep as it physically could into the material. Just prior to starting this toolpath, my son was zeroing the location and pressed down and the gantry went down as far as it could. It has done that twice with two different tool paths (although both were cutting a circular piece of material out. The first time it happened the collet stopped it from going deeper. This was on the first of four cutouts, the second time it made it to the four one, but had been gradually cutting deeper across all four. We were watching and stopped the machine.
It seems (at least in the last case) it is going deeper as it moves from left to right on the z-axis. Not sure what the first case was different.
We have a spoil board, with a secondary "cutting spoiler board" on top. Material is 0.5" thick, 5.5" tall by 19" wide. Using a .25 up spiral end mill.
We had been running all day, running about 7 projects (same in nature) over about 5-6 hours.
I have also noticed that as the gantry moves (left to right) across the x-axis I see the y-axis moving "away" (about .25" over 24"), so think it is moving at an angle. Not sure if the two are related.
Any help is appreciated.

[balbritton]

Which machine do you have?