Nobody knows for sure the reasons for moving those bitcoins and people can only speculate on this topic. News sites need hype to attract readers, so they create various rumors about hacking this wallet, for example, with a quantum computer. There is no evidence, only speculation. There is already too much noise on this topic. Whereas the reason for moving these bitcoins could be either a sale or a banal transfer to their own wallets. BTC-community is extremely sensitive to any news, which can affect the stability of the market.
And regarding quantum computers, while there is no proven fact of the possibility of hacking a bitcoin wallet with this, there is no reason to panic.
Quantum computers still look like another horror story for bitcoiners.
It is true that when large amounts of Bitcoin move especially from older wallets it creates much of theories with news sites mostly spreading rumors like quantum computer hacks for attention. However it is important to remember that these are just guesses not facts because today quantum computers can not actually break Bitcoin security. These movements are simply owners selling their Bitcoin or transferring it to different wallets for better security. Bitcoin community tends to react strongly to any news which can affect market but there is no need to panic about quantum computers posing immediate threat. They are still long way from being able to hack Bitcoin.