Alito hates Biden, who led the Senate opposition to Alito's confirmation. Alito has made fun of Biden in comments from the bench during Supreme Court arguments. The legal bloggers who cover the Supreme Court reported that Alito stayed away because he didn't want to be in the same room with Biden.
LOL -- Alito's not the only one. However, although opinionated, he's also normally very polite. To publicly "diss" the future-POTUS and VPOTUS, not once but twice, sounds a little unprofessional for a Justice of the SCOTUS.
I guess I just cannot see Alito doing something like that. IMO, he would have been respectful, just not enthusiastic, and been a contrite rent-a-crowd for the Executive duo. I just can't see pure disrespect and hatred as the answer for Alito's absence from both big D.C. galas ...