3rd possibility, 60 tons of 155 shells, propellant, etc was stored in the open. That is about 150 pallets of shells worth of stuff. Not at all unreasonable to move and store.
It happened at the battle of Roi Namur in WWII. A Jap bunker held about 100 torpedoes with 1000 pound warheads. A hapless Marine threw in a satchel charge and 100k pounds of explosives went off.
Just cannot see it being a nuke. And if a NATO nuke went off, Ukraine would howl that Russia had nuked them and NATO needed to come into the war fully.
That would have gone off like a string of firecrackers; what actually happened was one titanic explosion...