Most of those cases are due to user's actions, either they upload there seed online that someone got access to it, hacked their device (desktop/mobile phone), phished, then acquired their holdings and etc.

Some are just display bugs or server errors. While there are other cases that users can only accused their service or somewhat as inside job as stealing their coins, coz even though we were told that "blockchain.com don't have an access to users wallet" still there's no 100% proof for that. Though the app/website is somewhat open-source, as per said above source codes can be changed without no one's knowing.

That's why mostly don't recommended to use their wallet or any online wallet coz of risks of hacking, stealing and etc. If anyone want to have a peace of mind use electrum or any trusted hardware wallets such ledger and trezor instead.