Riker has every right to be condescending to his underlings! He's the boss. Also, by this point, he's quite familiar with the Worf (née Tasha) routine of suggesting a needlessly violent scenario, and needs to nip it in the bud.
Fundamentally, I'm far from convinced that a "why?" indicates Riker would be off base here. Even after completely ruling out foul play, the Geology Department would have a thousand "whys" left. Conversely, even if the events were
simple geology by all known standards, there's no way to rule out that it was all set up by a mischievous god, from the divine family that likes to insert fake fossils on the planets it creates and holy messages in randomly selected oil stains and backward-played LP records. It's about percentages and practicalities, and Riker would be in a position to say "The chances of foul play are so low that using this sentence for saying so is a waste of breath", so he doesn't.
It's fruitful communication at its best: Riker puts his underling on the right track with an injection of useful information and eliminates slack from his research machine.