but I am not aware of any other reason which could have lowered the SNRM.
When you see the SNRM value in the modem stats, it reflects the current actual situation, not the target. The actual value varies second-by-second, depending on the noise interference encountered. Noise can change, so SNRM can change too.
A low SNRM can be seen with a normal sync speed, for example, after a new subscriber is activated. In this case, the attainable speed will be lower than the current, normal speed. Next time there is a resync, the sync speed will likely be reduced.
A low SNRM can be seen with a high sync speed, for example, after a DSLAM has been restarted, or the area has had a power cut. In this case, modems can start with a 6dB target and a higher-than-normal speed but, as other subscribers start up in parallel, the SNRM value will drop. In this case, the attainable speed will be lower than the current (excessive) speed, but will be around the normal speed for the line. Next time there is a resync, the sync speed will return back down to normal.
In your case, it looks more like the latter has happened. The attainable speed (in the screenshot) was much lower than the actual speed. A resync put everything back to normal.