Could be one of many things, if you're going through a very good run of form which by the sounds of it you are your players could just be getting more and more complacent as the season goes a long until they hit a point. It's happened to me before and it's always usually when dominating, somewhere between 25-35 games played my team will lose it randomly and it hits them hard.
Like the others have said though, certain tactics will work a lot better against you than others, a mix of a poor tactical battle and players either being lazy or over confidant can easily throw your team off the rails.
I tend to have three formations now that are all fairly different and allow me to easily switch styles of play without causing my team to drop any familiarity other than the odd bit on certain things. There was a thread about doing the same thing before, I can't remember for the life of me the name of it but it honestly works very well once your team get used to it.