John is right, whats happening here is that somehow the ants have found themselves intersecting with part of the static world (which they are not supposed to do - that's the bug). They can't get a normal vector easily to push them out (normal vectors are easily found when sweeping an ant during movement, but once it is intersected it can be a bit tricky - based on geometries & centre of volume as well as having to worry about moving an ant into an open space that is actually inaccessible. Because this shouldn't be happening in the first place, I didn't go to the trouble of making a neat exit strategy and instead tell the creature to move in a straight line through the world to its next path destination.
So the movement through the floor/wall is an intended "emergency escape". Obviously it doesn't look great, but I need to go back into movement routines and find out why it intersected in the first place, then come up with a fix. Rather than risk breaking something else now, I think I'll leave this one until we are running a long beta test.
It's on my list now anyway, as always, thanks for the tip and the great slow- mo video