RJ85 Fire Fighting extreme near miss

RJ85 Fire Fighting extreme near miss!
Safecom report was filed August 1, 2019 by the crew of a BAe-146 about an incident that occurred July 1, 2019.
Here is the complete narrative from the pilot:
While conducting retardant operations I descended below a ridge crossing altitude. This was NOT on purpose. I tunnel visioned the drop, and continued down. This was a little fill in spot and I was really focused on finishing the line. As I stated, this was NOT on purpose. We{crew} debriefed and talked about what happened, and of course, how to prevent this type of screw up.
Source: fireaviation.com