The midfielder was shown a straight red card for kicking the leg of Cottagers defender Fernando Amorebieta off the ball and his decision to do so has been lost on his manager.
"I just don't quite understand where he's lost his cool and why it's happened," said Allardyce, whose Hammers side squandered an early lead.
"I know the centre-half fouled him, and I also know that (referee) Mark Clattenburg didn't give a free kick and he should have done.
"But it's still not an excuse for what Kevin did, and he ends up getting himself sent off."
A scathing Allardyce added: "So he's responsible for us probably losing the game more than anything else, there's no doubt about that.
"I don't understand it because this indiscretion comes straight after what happened at Liverpool.
"So I'm going to have to find out what's wrong with him, there's something wrong with his mentality at the minute.
"Because at his age and his experience, he's a player I've known for many years, I haven't seen this type of reaction or this type of situation that he's put himself into.”
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