I wouldn't say the Yankees are falling apart but it's a bit concerning how they have been playing since July.
I don't have an easy way to look at splits from a specific date, but since the all-star break on July 19 (20 games, not including tonight's loss to Seattle), New York is OPSing .756 which is still 7th best in MLB. But they're batting just .238 in that time -- which is 21st in MLB.
Their team ERA since the break is 4.36 (24th in MLB) -- though I don't know how much of that is the rotation vs the bullpen. It certainly seems like the bullpen has been shaky recently.
Those bats will have to wake up soon. While I believe that some regression was inevitable for the Yankees, and while I think they should be on pace to have a great season and possibly advance deep into the playoffs, slumps in baseball have a way of affecting a team's morale and that can sink them into deeper struggles.