Possibility 3) He wanted to discourage Republicans from going to conference: Reid has been accused of secretly sabotaging “immigration reform” before. Maybe he decided to do it again, not so secretly this time. Why? It could be he’d rather see no bill pass in this Congress so he can blame Republicans in the 2014 off-year election. In other words, “he’d rather have the issue.” This seems like a foolish gamble, though: Most observers think Republicans are going to do fine in the off-year election–and if they do fine after stiffing the Dems on immigration that will only make it harder to scare them into passing a bill in the next Congress. Reid might wind up making no Congressional gains and losing amnesty.
A more plausible sub-theory is that Reid only cares about maintaining his Dem majority in the Senate and thinks the immigration “issue” would work in enough key Senate races, even if GOPs gain a few seats in both Houses. It’s hard to see what those key races would be, though. (North Carolina? Only 4% of its voters were Latino in 2012.)
Alternatively, maybe Reid wants a bill, but he’d rather have no bill than a messy conference that a) might muddle the issue of which party is to blame for what and b) is unlikely, in his judgment, to produce an acceptable bill, assuming Boehner really has to please a majority of his own conference. So sabotage it now.