yeah, this is a common problem when drilling through most materials. Your standard drill bit is designed to route the debris the other way and back out the hole. It doesnt always work that way though, sometimes the debris can get back in the borehole and cause problems.
Either that or maybe they drilled through a 'soft' more unstable area.
In any event, usually it doesnt take a lot to 'cleanout' the borehole. Although I would imagine at those depths, things like that are taken to a new level. What I dont quite understand is how they were able to guide the tip of the drill.