A game has the following house rule:
"All players in a showdown must show all of their cards. You may not muck your hand during a showdown even if someone has already shown a better hand."
There is no "last aggressor" rule, or any rule regarding which player(s) should show first.
The following scenario occurred during a hand:
All cards have been dealt and all betting rounds ended. Several players are all-in while two players, Player A and Player B, are not all-in. Player A and Player B have their own side pot.
The dealer collects bets and announces "show". All players show except Player A, who pushes his cards forward face-down after seeing the other hands. The dealer physically turns over Player A's cards and announces the "must show" house rule. Player A's hand is beaten by Player B's, making him not entitled to any part of the pot.
Player A says the dealer should not have turned his cards over. The dealer again states the house rule. Player A says the situation was "not a showdown". No further comments were made, as the hand was already shown and the situation could not be reversed.
The house rules do not define the term "showdown". Is there an accepted definition of the term "showdown"? Was anyone at fault in this scenario?