Let's add # 3: an offer he couldn't refuse.
As for blackmail, one possibility is that whatever he was blackmailed about came from his FBI file, which could have been stolen by the Democrats in "Filegate" c. 1993. It's been reported that Bush 41 had been interested in appointing Roberts to a federal judgeship as far back as '92, so it's possible that the FBI routinely compiled a file on him as part of the standard vetting process.
For what it's worth, that's my take on it. Roberts was told he and his family would be killed in "tragic accidents" if the bill fell.
However Roberts, rather than simply going with the libs, got a little of his own back. He knew by calling this a "tax" that it was going to be an albatross around Obama's neck, likely to sink his re-election. That is why he could not explain his reasoning to another justice - there was no sound legal reasoning. Robert's took this path to keep from getting his family killed but, at the same time, handed the republicans' a huge club with which to batter Obama.
If he were worried about his "legacy", I think he would have given some thought to the fact that when people look back on this decision, they are going to say: "How in the hell did the court find it WASN'T a tax (for anti-junction purposes) and WAS a tax (for constitutional purposes) in the SAME DECISION!"