About one-third of Americans believe that 9/11 was an inside job by the Bush Administration. source: Scientific American.
The claim is that these trails that airplanes leave in the sky (if they are long-lasting) are chemicals being sprayed for unknown purposes. The craziness often includes such claims as fighting global warming, attempts to reduce the size of the population, the idea that the US military wants to use the weather in future wars, psychological manipulation, etc.