Can anyone explain what's happening here please

My game seemed to have gone to pot, if you'll excuse the pun. I spent a few quid on a two hour session with a World Snooker coach and really enjoyed it and picked up loads of good advice.

We were doing some basic short range straight potting blue off the spot into middle and every time the cue ball struck the blue the blue would hit the left knuckle and rarely go straight in the pocket. So first off to ensure I was hitting the white dead centre he had me knock the cue ball up and down the spots. No problem, straight up and down. So we went back to the blue and every time the object ball was struck it went left.

He said that I needed to readjust my aim such that it would go to the right of where I thought it needed to go. i did and that worked. he pointed out that it would take many many hours of practice to effectively retrain my brain. Im fine with all that and I continue to try and correct it.

But here is my question. for many years I had no issue with picking the angle and potting the ball. So why has this happened - what's going on with my brain/eyesight?

To be clear - Im not looking for solution to fix as I believe I have that, I'm just asking what folk's theory is to the root cause?

Thanks
Smee

Can anyone explain what's happening here please