I am pretty sure it was a defensive tenet for the Celtics that the guard(PG or SG) that was guarding his man on the court's weakside was supposed to cheat off his man towards the paint and into a passing lane. Saw Ray, Rondo, Bradley and Lee do it so much, it had to be by design. This allowed for the guard to be available to steal cross court, over the top passes, double quickly down low from the weak side, if necessary, or rotate across the foul line to cover a man in the three point area if the man with the ball strong side goes baseline.
So, yeah, Rondo cheated into the passing lanes a lot but it was because that's what he was supposed to do. Bradley, Lee and Ray Ray too.