For awhile I thought perhaps some animal lived in the log and that the cache was blocking the entrance to his lair so it would literally pop the cache out of the log like a cork.
If this was happening then anyone visiting the cache would simply find it lying out in the open. After logging the cache the tire might seem like as good a spot as any to hide it.
But so far no one has posted that this is the case. Only that the cache was not where it was described as being.