Looks like an attempt to get rid of the negative image associated with the name Singularity Institute.
From OP:
When Singularity University (SU) acquired the Singularity Summit from us in December, we also agreed to change the name of our institute to avoid brand confusion between the Singularity Institute and Singularity University.
I'm not sure this name change is a good idea or worth whatever SU offered (or that there was a real brand issue), but there apparently was some other motivation than 'SI now has embarrassing connotations'.
There is a real brand issue. I say "Singularity Institute" to people down the pub, the ones who've heard the word go "ah, Kurzweil!" (I was trying to explain this site I like called LessWrong.)
http://singularity.org/blog/2013/01/30/we-are-now-the-machine-intelligence-research-institute-miri/
As Risto Saarelma pointed out on IRC, "Volcano Lair Doom Institute" would have been cooler, but this is pretty good too. As the word "Singularity" has pretty much lost its meaning, it's better to have a name that doesn't give a new person all kinds of weird initial associations as their first impression. And "Machine Intelligence Research Institute" is appropriately descriptive while still being general enough.