Lots of causes.
Master socket, or rather the filtered faceplate, to start with - the latest Openreach design (for a company having so much money to screw up that design so badly is unforgivable) means that it comes off easily and can be easily knocked off. I noticed that if the filtered faceplate not right in, this will cause FECs and also CRCs as you fiddle with it, and that's not the first master socket with such issues
- Hoover being too close to the router can result in my Internet dropping and I've noticed certain powerline plugs can have a detrimental effect on the quality of the connection, too. For example, Devolo Mesh (rebranded Magic 2) caused lots of FECs and CRCs on my line than Magic 1. But this could be down to the way my wiring is.
- anything loose in terms of wiring, like lightning protection at the pole in our case resulted in SNR fluctuating and CRC errors. That's what Openreach told me, anyway, but whether they were telling me the truth is another matter.
This VDSL technology is very prone to issues, I've found.