Sometimes in the time before a power cut you get poor quality power and brief outages (brown-outs... when the lights will visibly dim for a second or so), which because of the cheap consumer-grade power transformers that are supplied with most pieces of electrical equipment, can cause havoc if the device is reliant on a nice steady voltage to operate properly.
So yeah, it's possible (in my mind anyway, but then again I'm no electrician) that your router dropped a few times in a short space of time just before the power cut or something, causing the line to appear unstable.
I don't understand how DLM works.. in fact I'm rapidly coming to the conclusion that it's buggy as hell, because there seems to be little consistency between different reports I've read on here and other forums of how it actually works. I myself had my SNR raised automatically even though my line was stable at 6db for months - and then after a small handful of resyncs it shot up to 9dB, then very quickly up to 12dB, again after just one or two resyncs whilst I was trying to time the sync just right to get the highest speed possible (no DMT tool tweaking for my DG834Gv2, you see).
And conversely I've seen cases where people have had to manually tweak their SNR UP because the DLM steadfastly refuses to increase from the default 6dB even though the line is dropping several times a day. Madness!
For people in your situation, there ought to be a route for ISPs to ask BT to get the target SNR reduced, no questions asked, if the user can give a legit reason for the unusual instability (i.e. factors other than phone line quality, e.g. power cut, new router, etc). But I doubt that'll happen anytime soon, I had hoped DLM would have been tweaked a bit better by now!