I can only imagine that it found/dug a path/tunnnel around the sand and washed all the crud from the top of the sand out to the pool. Maybe there is some type of overpressure relief.?
Can you open the filter and visually inspect the sand bed? are there channels through it or is it even?
Since it's working now, keep a close eye on it when vacuuming. Can you vacuum to waste? You'd waste the water but keep the crud out of your filter.