Thanks for the report! Yeah, Spidertrons use Unity's actual physics engine in an attempt to "right" themselves as you slice off their legs. The current build has their legs very "springy" and once they start to tip over, they sometimes fly right out of the map, and miss the failsafe teleport barrier that is under the map.
There is a bug currently filed on Asana for this issue with Spidertrons. It's under "critical" as of this writing, so hopefully some help is on the way!
-Cheers