What actually went wrong
Your back didn’t fail on the swing.
It failed on everything before it.
Golf, tennis, padel and squash all ask the same thing: generate force through rotation, repeatedly, often off balance, and frequently after sitting still for hours. When a back goes during a round or a match, the swing usually gets the blame. It rarely deserves it.
What normally happens is that the hips and thoracic spine stop contributing their share of the rotation, and the lower back quietly takes up the slack — for months, sometimes years, before anything hurts. The episode is simply the point at which the compensation runs out. Which is why resting until it settles and then going straight back to playing tends to produce the same episode again. Nothing changed except the pain.
"A season is not won in the six weeks after an injury. It is held together by what happens in the weeks when nothing hurts."