Presumably because he said something inappropriate to the linesman. We were all frustrated by the lack of a decision for the apparent foul on Browne, but I think coaching staff should be disciplined if they don’t conduct themselves properly, whatever team they represent.
As an aside, I’m not that bothered about a couple of coaches celebrating a goal at the end. It’s football and goals generate strong emotions amongst fans and staff (unless VAR is in use of course!)
I read somewhere KR got booked for going on the pitch to pull Marcus Browne away from the Linesman. So for consistency their coaches should have been booked as they went on the pitch, especially as they appeared to then coach/make tactical changes whilst stood on the pitch.