When houses "suddenly" move it's usually due to tree roots or water leaks or mining activity. I presume none of those applies here but the movement is relatively "sudden".
The only other causation I can recall is a whole line of terraced houses in West Bromwich that suddenly began to settle along their front walls. It was eventually diagnosed as dodgy footings,obviously, but finally precipitated by traffic vibration. I wonder if there could have been heavy lorries or vibration such as driving in piles round there in the past few months/years?