Maybe they wished they had preferred shares? There are better vehicles for this, worked out far in advance of the liquidity/change of control. You shouldn't be surprised by the actions or requests of a shareholder during an acquisition.

Does your shareholder agreement include a "drag along" clause? What percentage of shareholders are required to vote in favor of the acquisition? I would check to make sure they can't screw up the vote if you don't take their (a bit worrying) offer.