Looks very similar to the effects of a crosstalker.
The short, sharp increase in SNRM is a neighbours line resyncing, increasing your SNRM for a single sample as they aren't connected.
Usually your SNRM would return to normal at the next sample but it has dropped slightly.
Perhaps this other line has had their target SNRM reduced and is now making use of the D3 band that they weren't before their resync, lowering your SNRM slightly.
SNRM per band may show more detail.