Okay, so maybe it's a bit of a gross exaggeration, but the on whole it's true. Other than 'Red Dead Redemption', 'The Last of Us', and 'GTA IV', I can't think of too many great recent console games that were released over the warmer months. Take sports games out of the equation because that's based on seasons, and the majority of major releases come from September - March. Such as any
CoD, Halo, AC, Battlefield, Gran Turismo, GTA (V, III, SA,
VC), Skyrim (and Oblivion), Bioshock (2 and Infinite), Gears, Uncharted, and Dead Space.
Are there exceptions to the rule? Certainly. But more often than not, a game company will choose to release a game during the winter months (because people are inside and playing games) to maximize sales. If they think they have a great product, they'll want maximum sales, and will traditionally release it during those months.
For example, the highest grossing video game on current consoles, GTA V, was released in September. And a lot of the editors choices were for Bioshock : Infinite (a March release), ranking it as the best PC game of last year, the second best 360 game, and the third best PS3 game (behind The Last of Us, whose developer Naughty Dog had previously released every Uncharted game during the winter months). I'll take that 66% (from last year alone) and 75% chance (from Naughty Dog's recent releases) to the bank every day.
And then, you have to factor in when E3 and the Microsoft Spring Showcase are, which is late May or early June. A stuido doesn't want to release a game too close to these events, because the game teasers and new innovations will potentially steal the thunder of the game being released. Also during May and June are the bigggest movie releases outside of the month of December, which have people spending their money at the theater rather than the game store. A studio will also have to have a large and loyal fanbase (such as Rockstar or Naughty Dog, which monopolizes the Playstation market) to pull off a summer release. There's a reason that GTA IV, RDR, and Last of Us have been the major warmer month releases - they simply have the capacity to pull it off with their studio. I don't think Monolith Productions (Shadow of Mordor developer) has the star power to pull off a warmer month release, especially with the mediocre games they've released in the past.