Bad fuel isn't impossible. I'd just expect it to affect more than two aircraft. But if I also had two unrelated malfunctions in a fighter and a helicopter, I'm not jumping to conclude there's a systemic cause either. Sometimes lightning just strikes twice.

Now if two F/A-18s crashed, that's more suspicious and could indicate a systemic issue in F/A-18 maintenance or operations. But there's a large difference between a helo and a fighter. Different parts, different maintenance procedures.