Did you ever play football/soccer as a kid? If so, you'll know that there is a single formation and tactic that's used across the planet. Namely, everyone runs straight at the ball. It's a tactic that's drummed out of you by your teens, but it seems to have snuck back into FIFA 15 on the PC, and is now affecting some of the best players on the planet.
A number of threads have popped up complaining about the issue on EA's forums. The bug doesn't just cause all 20 outfield players to run into the middle of the pitch, but both keepers as well.
Note that this is only affecting the PC version of the game. If anyone's wondering, I've got the PS4 version for review, and will be posting that sometime next week.