Bal's right thats it pretty much automatic, as the penalty you accrue for declaring war is appplied to all regions on the map- except allies.
Theres the one way i know of to control this rush to war by everyone on the map when you declare war.
If you make some allies in strategic positions,those allies will NOT be likely to break the allaince and declare war on you.It can happen under very severe circumstances that they break their alliance and come after you, but you really have to earn it
So lets say your playing on the europe map and you want to take germany.
Make an alliance with france, austro-hungary and poland first.This way when you declare war on germany,even if you have zero belli against them,those three regions will be very unlikely to break their alliance and come after you.
You can than , at your pleasure pick them one at a time and break the alliance yourself with them, which will often allow them to declare on you.
Thereby fighting them one at a time, or at least in numbers you choose for the most part.