It is true- a lot of those patterns are deliberate. It's not exactly a free-for-all up there; usually, planes have to fly over specific airspace, and in the more populated sky must keep a certain distance from the projected paths of other planes, and possibly prohibited from going back over their own previous path due to turbulence and visibility hazards.
It is also true there are sometimes chemicals coming out of planes. Technically, there are always chemicals if you consider H2O, CO vapors. But in addition to spraying pesticides over crops, some planes occasionally dump their fuel to drop weight, which produces a vapor that is distinctly different from its normal exhaust. It's not talked about too much, but it's 'supposed' to be harmless, or not harmful enough for us to be worried about.
The trails will expand after a period of time being suspended in the air. This is just how nature works.
