They love water, (they are related to the willow), and this has been a cool and wet Spring. The trees are feeling their might and have put out a banner crop of seeds. It is like Dandelion Mania or something like that. Here is a badly focused pic of a seed, but you can get the idea of the filaments that carry it in flight.
I am watching thousands of these float by as I key this text. They are coming out of this scenario. Packed seed pods bursting forth in ripeness.
And they get collected by anything that can snag them, like this pokey prairie grass.