Air defense missiles do not carry large explosive charges and often do not cause aircraft to explode into pieces. They are designed to spray enough frag to foul up hydraulic lines, flight control surfaces, etc. which is all you need for a "kill" on a target.
Generally, if a plane is lost to mechanical issue it is related to power, i.e. engine, here it appears that the Su-34's engine remained operative. I am not very familiar with the Su-34 or Russian air force training but I would make the assumption that on a twin engine platform, their pilots have trained and rehearsed engine out drills and how to recover from the loss of one. That leaves the possibilities of a sudden sui generis critical malfunction in flight controls, pilot incapacitation, or hostile fire. I am unfamiliar with Russian ejection systems but based on the successful ejection of the crew, I would lean towards discounted pilot incapacitation. That leaves a loss of flight control due either to sudden malfunction or hostile fire. Given that the event took place in an active warzone with contested airspace, in which the Ukrainian air defense forces have previously shot down the same type of aircraft, it is reasonable to assess that this is a shoot down.
