I've heard of stories where people shot up massively like that too. It is more like a delayed reaction of learning, but not improving and then something just "clicks." In this example, maybe the last several thousand games before 18,000 they were learning a lot, but it takes a while before their learning starts to show itself in their games.
This rating climb is extremely rare though; much more common is gradual climbs over years where there are MANY ups and downs in rating and usually lots of drops in there.
https://www.chess.com/stats/live/blitz/sekey777/0
18,000 games, around 1450 blitz, then next 2000+ games up to 2400+ rating
Just wondering if anyone else has suddenly shot up in ratings like that, no accusations, I assume the person started taking chess seriously and studied hard.