I was playing earlier and managed to capture Jerusalem as King of Georgia. Within days of making peace with my former nemesis, the Emir of Medina, the DOW from western Europe started rolling in. I suppose it's not really an implausible turn of events but it wasn't really the direction I wanted the game to go. The English had a claim on Jerusalem from having conquered the territory earlier on in the game but it wasn't them who declared war. The HRE was the first to jump in along with several of his vassals and then France along with a few of her Dukes jumped in.