I think it is due to an accumulation of new lines being connected.
I was one of the first (if mot the very first) to be connected to the cab back in June 2011.
My intermittent line fault was finally fixed in May 2012.
I then enjoyed DS sync speed of around 30 Mbps & a very stable connection for 7 months or so.
I started seeing stepped deteriorations in SNRM from January 2013, always followed a few days later by a resync at a lower speed.
Unfortunately I don't know what my Signal attenuation was back then as we were using the original HG612 modem firmware that always reported the D1 band Line & Signal attenuation as identical values.
The damage had already been done gradually by the time the firmware was updated in October 2013, which was the first time we could see any differences in Line & Signal attenuation.
We do occasionally experience power cuts lasting for just a few minutes.
On these occasions, sync speed/attainable rates do improve a little when the connection syncs again when power is restored.
My 'assumption' is that as the HG612 syncs up quite quickly, it manages to be up & running before some other users' connections actually sync (maybe slower to sync ADSL modems?).
However, syn/attainable rates get nowhere near the previous glory days of 30 Mbps, so another 'assumption' is that the main disturbance is from newer VDSL2 connections that also sync quite quickly.
As I'm around 1100 m from the cabinet, it's also quite likely that quite a few other users have their power fed from different sub-stations, therefore they have probably remained in sync during the power cuts local to just my area.
EDIT:
FWIW, I have attached a graph showing 653 days worth of sync speeds (occasional temporary slight respites (probably lasting for a few days after a power cut).
My connection is currently in sync at 18.68 Mbps.
I do have earlier graphs where it was in sync at almost 35 Mbps, but I haven't included that data as they were only temporary fastpath speeds following DLM resets, until DLM kicked in again, applying Interleaving & therefore reducing sync speeds.