You can - Mobile or PC tap the location and see what the (-) % are from to have an idea on why the value is so low.
But generally speaking:
There are a few ways to help the back lash of expansion - that I have found.
1. Presence - allowing your arms to rest in place and safe guard the new lands.
2. Rebuilding - if this location had or has buildings in damage - rebuild to help rebuild moral.
3. Resources - ever acquisition brings on a debt to you as you will now have to up keep this new land. This can and quickly change you resource abilities - monitor and adjust as needed.
4. Time - Once the above are stable - give it time: I highly suggest (or found that even brief breaks between expansion) has maintained over all #/% far better then just sweeping land.
Other factors to think of that may apply to one map/ war then another is:
Capitals - Expanding will happen and a planning this can be very important - the more center you are; the better.
Global Pop % - will create an impact on the % morale of territories based on the neighbors.
