Maine has had more rigorous standards for minimum lobster sizes and not taking egg-bearing lobsters than neighboring Canada and New Hampshire. Not sure about now, but this was the case in the past. Lobsters don't stop at boundaries, so this was beneficial to Canada & NH lobster fishers.

Warming. Check. Gulf of Maine is warming faster than virtually anywhere on the planet.

Over fishing.... Nope. I know most people don't have literature access, so here are some quotes from: Ryan, Richard W., Daniel S. Holland, and Guillermo E. Herrera. "Bioeconomic equilibrium in a bait-constrained fishery." Marine Resource Economics 25.3 (2010): 281-293.

"The massive amounts of herring used for lobster bait are hypothesized as one of the reasons for the unprecedented productivity of the lobster fishery in Maine, where landings in recent years have been more than three times pre-1990 average levels (Grabowski et al. 2010)."

"The Gulf of Maine lobster stock is the recipient of a “growth subsidy;” i.e., increased growth as a result of consumption of bait. Jury et al. (2001, p. 1127) find that of the lobsters entering a trap, only 6% are captured, whereas 94% escape. Among that 6%, only legal-size lobsters would be kept, while the others would be immediately released. Due to the high rate of Bait-Constrained Fishery escape from traps and the fact that sublegal lobsters along with V-notched5 females are released, bait consumption while in the trap provides a free meal for many lobsters while they grow. Additionally, discarded bait supplements food available to wild lobsters. Saila, Nixon, and Oviatt (2002); Grabowski et al. (2009); and Grabowski et al. (2010) attribute the proliferation of lobster biomass and landings at least in part to these externalities associated with the process of harvesting lobsters. In a mark-recapture study, Grabowski et al. (2009) find that lobster in areas with traps; that is, areas delineated as open to harvesting lobster, outgrow those in areas without traps by approximately 15%."

With herring getting expensive, lobster harvesters have turned to other bait. I strongly suspect the net input of biomass into the Gulf has increased since more of those sources have become freshwater (see the commercial sucker or sea-run alewife fisheries in Maine). Essentially, the Gulf of Maine is a lobster farm that's had increasing numbers for decades due to the warming ocean and the increased input of bait. Now it's starting to reach the point where it's too warm and the lobster are moving North. Catches have been increasing Downeast. Over the next 10-20 years we'll see Southern and Midcoast Maine catches continue to decrease while Downeast and Canadian catches increase. Additionally, as others have pointed out, there is nothing natural about the Gulf. See the previous collapses (ie, cod, shrimp, herring, scallop). The warming is also presenting as species moving into our waters. For instance, I have little doubt there will be a decent recreational blue crab fishery within the next couple decades.