It's a design issue compounded by population imbalance. Now as far as Trelle goes, it seems it's not really the fort we've been taking the most:
http://rostatus.heroku.com/world/horus
Maybe the fights there last so long and drawn out that it's being singled out here. Based on the above, Syrtis is more interested in attacking Ignis (at least in my clan a lot of us are more interested in fighting in Ignis) for at least the last 7 days.