Driving forces can increase and therefore "overpower" resisting forces in several ways: a new snowfall, wind action, rain, cornice fall, the passage of animals, or... people.

Here, this is an important detail. 

If the first person has passed alone and perhaps with some caution to "probe" the snow, they have certainly caused an increase in driving forces at that point, but not enough to overcome the resisting forces; therefore, no detachment.

But if this single, consequence-free passage then results in a "free-for-all" approach for the group, according to which there is no danger and everyone believes they can do what they want, driving forces can increase considerably for the greater number of people who more or less simultaneously tackle the slope, uphill and downhill.

Additional loads, progressively uphill or simultaneously downhill, can affect the snowpack, and in this case, the consequences could be very different!

However, it is not just a matter of how much weight is put on the snowpack to trigger a detachment; we must also consider the aspect of the weak points of a possible slab.

In uphill skiing, everyone normally follows the track, but in downhill skiing, especially with beautiful snow and inviting slopes, skiers are inclined to seek their own patch of snow to make their descent first (Importance of the Human Factor!).

This increases the chances that someone will touch one of the weak points from which the fracture of the snowpack can propagate.

On the other hand, it is reasonable to believe that a heavily tracked slope has been stressed and tested in multiple points, and therefore any weak points should have already been touched by those who came before us.