Depending on where they were riding mountain bikes, I can totally see this happening. MTB trails can be very technical and dangerous for beginners. When I first got a MTB and had only done fire roads, I went onto some intermediate trails with a friend and fell a LOT. I was lucky I didn't get seriously injured, but I did get whiplash and some minor injuries. I then scaled back to fire roads and beginner trails only. The roots, rocks, switchbacks, and drops of some trails are not for the uninitiated! And let's face it, some of us are better bike handlers than others. A newbie is not likely to be able to anticipate how to handle even the smallest obstacles.

My DH broke his clavicle on his road bike when he hit a bad edge on the pavement, and he was a very experienced rider with many 1000s of miles under his wheels, so it can happen to anyone, but a first-time inexperienced mountain biker is much more likely to run into trouble if s/he rides on terrain above his/her ability. Sounds like that may have been the case here. Poor guy -- owie!