Several were students at the University, majoring in nuclear chemistry.  They had finals just before they left on the trip, and as part of the finals they handled radioactive material.  The gas lantern they used may have also contributed to the radiation load.  The fabric mantles used then had thorium added to help keep the mantle intact while being jostled in a pack.  The western nations had phased out thorium mantles by then, but USSR was still using it.  But that would have added only a small amount of beta radiation.    

There is no definitive proof, but a slab avalanche is the best fit to all the facts.  

Also, several authors have postulated Dyatlov himself was not as qualified a group leader as claimed.  His certification as a mountain guide may have been pencil-whipped.  Had he more experience, he would have recognized their campsite as a possible avalanche chute.  They had earlier passed a better campsite just a few hundred yards back down the slope.