Huge fire when joining server

We run a vanilla 1.14.2 Server and almost everytime when someone joins the server after there was nobody online for some hours, you can hear a huge explosion/lightning (sounds like there are multiple thunder sounds played simultaneously), and there is a huge fire in one of our bases, mostly in the same area.
the "spawn lightning", as we call it, appears independent of the current weather conditions.
griefing or trolling can be excluded.