How someone get 10x his army size as 'event'

[RafaMonteiro]

OK, im trying to come back to CK2 after years without playing and in the first few years of my game I declare war on a dude and suddenly he has 10x as much soldier that the game said he would have and they are 'event' troops?

how in the world I would know when to attack someone if they can randomly get this much stronger??
http://imgur.com/a/uAIN9

If someone can explain the logic behind that I would appreciate

 

[Ultima_Ratio]

Tribals can raise a 2500-man army for 500 prestige.

 

[Ternega]

It's not an event. All tribals can dump 500 Prestige to get 2500 troops. Troops vanish when rulers stops being at war for more than a day. Paying attention to enemy prestige can help deal with the issue. If you are in an offensive war against that, try to assault their holding or simply white peace if you can.

 

[RafaMonteiro]

Thank you for answer, its hard to come back so much is different

 

[Tatterhood]

I think tribal armies have been around for a long time, it's just that the latest patch made the AI actually use them.

 

[hagagaga]

I think tribal armies have been around for a long time, it's just that the latest patch made the AI actually use them.

This is correct. It was in the patch notes.