I'm sure it's all fine, with the tank being mature and all, however with any fish deaths the first thing I'd do is test the water.
One symptom of skipping water changes can be a drop in PH.
A dramatic PH drop could end up killing some of the Nitrifying bacteria in the filter, which could, in turn, lead to a build up of Ammonia and/or Nitrite.
If the filter was cycling again then even a water change would only be a very temporary fix until the cycle was complete.
I had an Ammonia Spike (caused by me adding too many fish to a recently cycled tank) - Ammonia went up to almost 3ppm in 24 hours.
I performed a large water change and tested it shortly after and it was 0.25ppm. These kind of levels can significantly affect the well-being of your livestock.
24 hours later and it was up to about 1.5ppm again. I did another large water change and continued doing so for 4 or 5 days until the bacterial colonies grew to a level where they could cope with the additional bio-load.
The point is, I'd have known none of this without testing the water, and I would have lost fish.
I'm not saying that is what happened to you, but eliminating Ammonia and/or Nitrite from the equation is the easy part, then, with that out of the way, we can start thinking about what else could be causing the deaths.
I'd rather know what it isn't and go from there instead of presuming and guessing.