You sell something, they deduct the taxes.
You get dividends, they deduct the taxes.
If you may owe tax on any transaction, they will deduct it. It's quite a contrast with a US vanguard account where they don't deduct anything and let you pay it when you do your taxes.
The thing I really don't like is that when they tax your dividends, they deduct the funds from your account, which may be accumulating tax free, and don't let you use funds from outside the account. They may have a way to do that, but I just moved that money away from Schwab.