They do the same thing here in the Atlanta area. Its always several planes criss crossing each other in a grid pattern. If you watch them all the time you can see them turn the spray off and on. I saw a plane the other day and while I was watching he turned on the spray and started a trail.
It drives me crazy that, even with good binoculars, I can’t tell anything about the planes. They are just unmarked white ones. Certainly not the regular commercial and military ones that have always flown here.
That is the exhaust condensing at certain temperatures and not at others. Just like car exhaust.