It’s real simple. Some Trump adviser told him Rubio is the one rising and threatening to derail him in NH, and it would not help Trump to keep attacking Cruz.
I think, and hope, this might be the influence of Jeff Sessions. That Sessions staff member went to work for Trump just a few days ago and he also issued a very good statement for the Trump campaign, yesterday, I think.
I believe Sessions might be moving more towards Trump after his warnings this week about TPP which was just signed in New Zealand with no publicity. Trump has been strong against TPP and Cruz entirely unreliable after his vote for TPA.
Rubio needs to go down and down hard.