Hopefully this is fixed in 0.915 but just in case it is not I'm posting it.

Against my better judgement I agreed to a trade route and road to my capital with the Tarth (they offered me a lot of money).  A few turns later a train of umberdoths appeared (2 packs of 3 and 4 singles) heading for my capital following the new road from the Tarth.  Presumably they had been milling around the Tarth capital as there were no lairs near my border and had been aggro-ed by my caravans following them all the way home.  I survived the onslaught (just) but had to leave a stack at the road to deal with occasional stragglers.