snrm db is a variable value, there is a target of 6db but that only applies at the time of the sync event, after that point the db value will move around dependent on noise on the line. crosstalk is one such generation of noise.
I have never known for DLM to take action on a line simply for have a low snrm, however as snrm drops the error rate can increase which in turn can cause DLM action as DLM does act on the MTBE (ES count).
It may also act if there is too many loss of sync in a 24 hour period, in particular if these occur close to each other.