The exact same thing happened to me today as France. I was in a war against Ferrara (leader), Austria and Tuscany, it was the "Ferrarean Conquest of Venezia" to which Naples had called me and I became war leader of the Defense. After crushing all opposition I vassalizes Ferrara and was immediately without CtA, Popup or anything in a war against Brunswick and Magdeburg (Brunswick was trying to Conquer Oldenburg). Again I automatically became War leader in a ridiculous alliance with my nemesis Austria and half the HRE. The next day I received messages that all my allies had "backstabbed" me and because of the -150 Relation hit I could not re-ally them.
Nothing that was said above in order to justify this mechanism makes any sense to me. When I vassalize some small dipshit OPM, it should pull out of any wars it's in and automatically be in MY wars, not the other way round. Regardless, there should be no autocall on my allies (which would have happened if I had been attacked, but I wasn't I was just joining a defensive alliance).