I'm living in town house where two houses caught fire due to the presence of methane gas. Is it possible that the reason is due to gas accumulation fron the gardens which surrounded by trees with its roots propagated horizontally

I don't know if that is possible, but I personally don't think so, unless those two houses are completely closed. I don't think the rate of production of methane by trees is that much and if I'm right, then the methane gas would be daily swept away from the house if well ventilated.

It is best to make tests for methane gas though. It can very well be due to a leak from a pipe... have you tried to measure the level of methane in your house?