Well, IBM certainly is no stranger to anti-competitive behavior. If SCO had focused on breach of contract from the beginning, their case actually would have had some merit, IMHO.
But then they went bananas and made fools of themselves with the whole Linux-copyright-violation bit, destroying their case more thoroughly than IBM's lawyers could have done.
As far as I know - which isn't very far! - Microsoft started to support SCO only after they had dragged IBM to court.
SCO had pretty much bet the future of the company on their and IBM's joint Itanium Unix, and when IBM pulled out of the project, SCO was rather screwed[0]. Suing IBM for breach of contract was a sensible decision, as far as I can tell.
[0] Considering what a phenomenal failure Itanium became, I suspect they were screwed either way.
It seems Microsoft did funnel money to SCO by first agreeing to pay a $16m licensing fee to SCO for something that they fought against having to pay in the 80s and for which SUN only paid $9m. They also gave $50-60m to an investment firm which intern gave it to SCO.
Beyond financing the fight, there hasn't been any other evidence of their involvement. Anything else is just conspiracy theories.
Microsoft was certainly in full "Linux is a cancer" mode at the time, and even had an executive, Martin Taylor, who was essentially their Linux attack dog. That said, while they were certainly not opposed to what SCO was doing, there's not much evidence that they were the man behind the curtain either.
But then they went bananas and made fools of themselves with the whole Linux-copyright-violation bit, destroying their case more thoroughly than IBM's lawyers could have done.