The only reason to sign anything is for an agreement to keep this under wraps, or something in your favor. If it's NOT in your favor, you have nothing to lose by walking out without signing anything. That signature is for THEIR benefit, not yours, so don't bother helping those that fire you. Hell, then you can tell your next employer something to the affect of you being fired for not signing some BS agreement they insisted on. It can't be discussed, so it may win you a favor and still partially true.
Could you have possibly been at a party and ate some snacks, say, brownies, that might have been tainted without your knowledge? I was at a wedding and a fella was nice enough to spike my punch without me knowing. I was nice about it, but it pissed me off since I don't drink. I dunno, just tossing out some thoughts...