Weather seems to be gentle so I doubt it is related to that. A collision seems unlikely unless the crew of a plane with a huge radar dome stuck on it is as incompetent as the ground radar crews are.
Theoretically all it takes is putting part of an old Kub or Osa launcher on a non-descript ship. The waters around Crimea aren't exactly water tight as raids have shown and I'd expect both the Sea of Azov and the Black Sea coast to be even worse. Both aircraft have zero countermeasures.
My guess? Captain Pricekiy snuck ashore, commandeered an S-400 battery and let rip!