Maybe someone used summon monster targeted the middle of a suspected mine field to see if any of the badgers would blow up a mine? (What happens to the corpses when the spell duration expires anyway?) At least they weren't fiendish dire badgers. :)
Assuming that the badgers are summoned (e.g. by a Summon Monster spell) as opposed to called (e.g. by a Planar Ally spell), they disappear at the end of the duration or upon death/0 or less hit points, so no corpse is left behind (and, in fact, they aren't actually dead, just returned to their plane of origin). If the badgers are called, then they die and leave a corpse, just like normal. This is explained in the magic overview section of the PHB when it describes the Conjuration subschools. (Incidentally, normally extraplanar badgers are Celestial, not fiendish, although I suppose they could be.)
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--Adam