[georgyo]

Or the simpler answers, something on your computer reveals it, like malware.

Or the person who sent/received money to/from you revels who you are, or information which helps identify who you are.

Or there are warents authorizing authorities to take your devices and then they have all your identies.

Also when you send Bitcoin from a wallet that has received money to multiple addresses, you can't be sure which coins are being sent. As such they can also associate multiple addresses together by which coins they sent.