[Phantom Lord]

Doesn't require any collusion -- A is happy with his new price, B sees that A is sticking with it and is happy with his price, C sees.......

And small company D wanting to gain market share drops the price forcing A, B, and C to follow suit or lose market share quickly and substantially to D.

[expatpat]

And company D realizes that his little price war didn't help his market share at all, and ruined his profits So he decides he liked the previous higher margins better than a bit more volume, and D,C, B, and A all go back to the higher prices.