What it comes down to . . .
The fruit ripens until the pods, now dry, burst like popcorn, presenting their seeds for the wind to distribute. Nature has scheduled this part of the process to take place over several days for each tree, timed to coincide with the peak of spring runoff, or just after the peak, so that silty places along the banks will still be wet enough to satisfy the seeds' immediate need for water. The florescence in the middle of the photo above has surrendered nearly all of its seeds; the upper one has bided its time.
On windy days the result is a blizzard of cotton. This tree may well be losing ground along Montana's rivers and streams, but in urban settings it can be such a nuisance that many cities prohibit the planting of female cottonwoods.