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It's mostly the demand, the whales are the specific person that do those because of course they buy a lot of coins.
Sometimes hype do as well in some coins. See those other coins getting pumped and dumped? Yeah they're the ones for hypes mostly and just die throughout the night.
Whales are mostly responsible for the sudden pump and dump of bitcoin price. Normal investors too like us can also make the price increase by setting a high demand for bitcoin and let other institutions or investors add to its demand. But whales too can manipulate the market by selling their bitcoins which will result into a dump market. So imagine how volatile the market is but still we keep on investing in bitcoin because we believe its high potentials to create huge returns in the future.