You didn't mention the type of brake controller. Is it a time delay or proportional? If it is a TD and sync switch is set too high, you could get an initial aggressive brake from trailer.
1983 Jayco 1006 Pop-up (antique status :hihi:)
2006 Jayco Jay Feather EXP Baja25k
2001 Sundowner 40' Horse Trailer
2002 Ford F-350 XLT Crew DRW 4x4