Delta moves flights early when headwinds are worse (or tailwinds weaker) or other issues (e.g., routing around weather) make the flight plan fall outside the buffer range provided in the block time. Because international flights are longer, they are more likely to be impacted by abnormally strong or weak tailwinds and so they are more frequently pushed to leave earlier. You will also see them held at the gate (despite no ATC congestion) when the reverse happens to avoid creating gate congestion issues at their destination - although they still "push" on time, presumably to avoid it hitting their departure statistics (probably annoying if flying out of a hub for folks with tight connections).

There could be other ones, but this is the reason why. I see it all the time flying to/from the West Coast when the jetstream is acting funky. From personal experience - the Wednesday/Thursday before Thanksgiving was one of those cases (virtually no tailwind from Southwest US to Atlanta) - departed 10 minutes early, took off 3 minutes after scheduled departure time (i.e., no abnormally long taxi time), landed at ATL with a short taxi, and we just got to the gate exactly on time. Wouldn't have made it on time if we hadn't pushed early.