Be at it again
Definition 1 of 1
Pro Tip 1/3Common Habit
Refers to a person returning to a usual pattern of behavior, often negative. 
After apologizing last week, Tom is at it again, leaving dirty dishes in the sink.
Pro Tip 2/3Negative Connotation
Often implies disappointment or annoyance at someone's repeat actions. 
The neighbors are loud at night, and they're at it again despite complaints.
Pro Tip 3/3Informal Usage
It's an informal phrase, used more in speech than in formal writing. 
I can't believe she's at it again, bailing on our plans.