In deep coal mines there are no toilets so a worker caught short needs to find a place off the roadway to drop his - errr - load. Normally such unused crevices are not big enough for a man so the miner shits on a shovel so it can be dropped out of the way. If it is near the end of the shift this can lead to loads of "fun" as the worker may decide to use the shovel as a mini trebuchet and fling the poo long distances at high speed (hence the phrase "it went like shit off a shovel," though steam train drivers and firemen also claim that honour). Sometimes, to share the fun, the shovel load is thrown at one of the speedy ventilation fans and the cry of "where were you when the shit hit the fan" becomes an accusation.

Older generation nurses in the UK modify the phrase "like shit off a shovel" to "like shit off a blanket", partly as a comic parody od "sticks like shit to a blanket" and partly because flicking the blanket hard was one way to remove lumps of contamination.

Interesting history it has, nonetheless. Maybe write to the owner of the etymology site and let him know. It would be interesting to see his quote change

alleged comment by one of the astronauts when seeing the toilet on the space shuttle.