Okay, as a new player, I'm not sure how strange this is. It may happen all the time. But it's the first time it's happened to me, and it seemed pretty damned odd...
I had a slider move available, and since I'm playing Novgorod and want to Westernize, I moved the slider towards Centralization. This particular slider move spawned a Pretender Rebellion in Bryansk. Not a problem, since I already have 10 regiments garrisoned there since it has a high revolt risk already.
So I'm watching my little army dudes fight and nobody is near close to being wiped out or running out of morale, when suddenly I get the "You won!" message. Well, that's weird, but okay. So I hover my mouse over the units to see where the rebels are retreating to so I can hopefully head them off. And I see this:
There's a Muscovite Nationalist uprising next door to me in Golden Horde-controled Tula. A few regiments of the Muscovite Nationalists came over to Bryansk, and I guess they don't like the Novgorodian Pretenders, because they actually started fighting them instead of moving on to seige another one of my provinces.
Well, the Novgorodian Pretenders may want to stage a coup d'etat on the current Novgorodian president, but they're Novgorodians through-and-through, and aren't about to let some upstart Muscovites get their filthy hands on legitimately-held Novgorodian territory. So they switch sides:
The Novgorodian Pretenders spawned with only five regiments total. Those soldier numbers for the Novgorodian Pretenders are his forces as well as my (legitimate) Novgorodian forces combined against the Muscovite Nationalists, because the enemy of my enemy is my friend.
And finally, we win the day:
Note the temporary alliance -- my shield and the Pretender shield -- in the "Win" side.
And after that, the Novgorodian Pretenders marched on Tver and were put down by legitimate Novgorodian troops, while the Muscovite Nationalists were vanquished in Vyazma.