Try thinking of it in this way:
There's a problem with climate change that is considered man-made. That's what used to be called 'Global Warming'.
The best 'fix' to this problem is to convince all the countries in the world to reduce their greenhouse gas emissions, among other things.
Countries are not really moving on that very fast, and people began to get worried.
Some people - scientists, governments, and so on - began to think up possible 'plan B' options to help mitigate Climate Change until we're able to get Plan A (emissions reduction) up to speed. This in case things start to go really bad with the climate, and emergency measure might be needed.
So, a lot of people began writing down and discussing theories on what 'Plan B' might be.. a 'quick fix' so to speak. There had already been a lot of theories about this, especially in science and science fiction, for a long, long time. People with imaginations and intelligence could see this situation coming for a while.
A lot of these theories, plans, studies, simulations, documents, etc. made their way out to the public.
Some people who read them saw that they were just that - plans, studies, theories.
Some other people believed that it was evidence that it *was already happening*.
Some of those people read about 'spraying' aerosols in these papers, and thought that because airplane contrails look like 'sprays' to the untrained eye, then these plans must be in effect, and were being carried out by all those planes and trails we see in the sky.
However, those planes had been emitting contrails and persistent contrails for a long time, even before these theories came up. There may be more contrails now than in the past, but there are also a lot more flights now than in the past.
People were just not generally looking up, noticing them, and worrying about them - until some of the people who mistakenly believed these contrails were part of a secret program *told* them to look up, notice and be worried.