Combustion engines are glitchy. Server lag can make them blow up, not getting enough water, world anchors not loading chunks properly, etc... Any of these things can cause them to blow up. This is why I made this video that shows how to run engines without water safely:
By cycling two sets of engines, they never get hot enough to explode. The only other sure alternative to prevent an explosion is to babysit the engines, which defeats the purpose of automation to begin with.
p.s. I just added the link. I forgot to put it in. :)