MP, best hand south, instant, pro gib 30
Another example of GIB freely bidding game having initially denied game interest, presumably on the strength of oppo bidding.
I suppose that if West can have 5xH for the 2C bid he might have had a Spade fit instead.
My guess is that it is unlikely, given that South's double of 2C is looking for a fit in that suit among others.
KJx of diamonds looks more useful in defence than offence.
So with all indications pointing to a misfit, East takes it upon himself to bid 2S. A spade fit is unlikely, he could not be flatter, and does not even have the decency to have a fit in the one suit that his side has bid and raised.
West's 4S bid was I suspect to punish his partner.

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ing) tr. v. - Any bid made by bridge player with which partner disagrees.