The exchange has suddenly decided you line is bad and set a min SNR of 15dB with a corresponding lowering of sync rate.
Welcome to the club. There have been many cases where a perfectly good line has been classed as bad by the exchange and there is bugger all you can do about it - I know, I tried for weeks to get the SNR reduced back to 6dB after it increased over a week from 6dB to 15dB on my line without any success. The only reason I did not pursue this is I was moved to LLU. I'm now fighting to get the LLU min SNR of 12dB reduced to 9 or 6 which I know my line will take, and this has been going on for months now.
Rate adaptive connections are fine and can produce some health increases in speed, when it goes wrong, as in your case and mine, it a PITA to get BT to accept there is a fault and correct it.