I think there are a number of interpretations of what this avalanche is , or was . There are many different types of avalanche and some of these types of avalanche couldn't have happened at that location. However, it is not impossible that some sort of snow movement or collapse of snow couldn't have happened.
There only needs to be a build up of fresh snow on the slope above the assumed trench they cut out to pitch their tent, that is, if we can assume that the last two photos are of the actual final tent location.
We can see from many of the photos and modern videos that the consistency of the snow varies from the top of hill all the way to the forest. Sometimes it's hard and it's like walking on ice and other times it's soft and people sink into the snow.
If we take the raised foot prints for example , we can assume that whoever made them, the snow at that time , in that area, was soft and the snow level would have been higher than the top of the remaining raised footprint. Later wind and snow blew that layer away before any searchers arrived .
So after three weeks of different weather conditions, a mass of freshly fallen or wind blown snow on the night of the incident could easily have been swept away. If it is the case that the trench was cut ,as depicted from the last two photos , then in my mind, it is possible for that cut in the slope to be a trigger/tipping point for snow above the lip of the trench to pour into that area.
I have no idea what kind of avalanche that would be called and I don't suspect it would be very big and certainly not big or strong enough to break bones. However, I think it would be big enough to collapse the tent and level the slope. I don't think it was a life and death situation ,if this is what occurred but it didn't need to be. Glennm explains that and I have to agree, the hikers just needed to think it was more serious than it actually was and to move away from further potential avalanche.
It is quite possible the hikers didn't know exactly where they were or had pitched their tent, it was late afternoon and from the photos the visibility looks poor. They knew there was steeper slopes in that area but could they be sure that they weren't sitting right underneath the steeper slopes? It is reported that they pitched in a safe, low risk area and everything was done by the book and that seems to be true.
But here's the difference, the difference is between knowing and not knowing, if you're in a tent and snow collapses the tent your sitting in , and you don't know or are now unsure of your actual location relative to the potential danger if you are in the wrong place , what do you do?
Most likely it is night with possible wind and snow, you are in a state of unpreparedness, clothing , equipment etc. One to two torches at best , do you head for the treeline to make fire until daylight to access the location or stay in a now , unknown not so safe location.
The reason to leave the tent may just be that , the unknown , "the unknown compelling force".