My theory is this: New Brazil president Bolsonaro (he came to power in 2019) wants to grow the economy. He sees a lot of potential in growing the economy via agricultural production so he encourages more transforming of the rainforest into farmland than is normal.

China usually buys most of their agricultural products from the US, but the trade war means they have to look elsewhere for soy beans, pigs and all sorts of other stuff. They look to Brazil to fill this gap in their domestic demand, which only increases the demand for Brazilian agricultural production.

So there you have it: US-China Trade War => Amazon fires