Welcome to the offseason.
Last season, there was some concern over how the New England Patriots were able to in the opening coin toss at a ridiculous rate. At one point in the season, the Patriots had won 19 of the prior 25 flips. Maybe there is magic involved; with Bill Belichick, no one can really know.
The Patriots were involved in 24 coin tosses in 2015 (4 preseason, 16 regular season, 2 overtime, 2 postseason). I was able to find the results of 23 of them (if anyone can discover whether the Giants called heads or tails in week 4 of the preseason, it'd be appreciated).
The road team is responsible for declaring heads or tails at the toss and they called heads a whopping 20 out of 23 times. The only exceptions were the Week 1 Steelers, the Week 8 Dolphins, and the Divisional Round Chiefs. The Patriots called heads in each of their 13 attempts.
It can't be a coincidence that all teams are opting to call heads at the coin toss. The results are definitely a coincidence.
In total, the coin tosses led to 12 heads and 11 tails, which is what you would expect from a balanced coin.
However, when the Patriots were in charge of the coin toss, the coin came up heads in 10 of 13 flips. When a road team called the toss, the coin came up tails in 8 of 10 flips. Unless there's a magnet at midfield, Gillette was also home to some serious luck for the Patriots.
Unless there's an issue with the official coin (which would be a serious problem for the integrity of the game), its likely that teams are opting not to worry with the toss and just go with "heads" because it's the first option.
Alternatively, there's a study from Stanford that shows "that vigorously-flipped coins are biased to come up the same way they started" and I'm 100% serious. Players could be coached to call whichever side is facing up, and officials probably also flip with heads facing up on a regular basis.
Whatever the reason, more teams are calling heads at the coin toss. Maybe they should start calling tails at Gillette.