Yellow acted unusually—he merged all his armies into one.
Are there no earlier saves?
Added after 11 minutes
I looked further. One yellow dwarven Castle isn't built up at all. In two others, the monthly growth hasn't even been bought out (there are another 20 dragons there, by the way).
Only Rolf's castle has been bought out. All portal guards are defeated. Hangvul is without an army, another yellow one underground is without troops, and one more was probably defeated. Everything is fine with the other two Clans, as it should be.
Hmm... It's amusing that Rolf doesn't only have dragons in his army; there's also a crowd of Jarls and Priests. I get the feeling he used a cheat for ten-fold town growth ;)