When we spend an old UTXO with a new one, it's telling everyone you're an OG, which can mean you have lots of Bitcoin, for example.
A behavioral fingerprint pattern would be, for example, receiving $5k from your boss every 1st day of the month, at 2pm. This can leak that it's your salary when you spend it, it can review which company you work for depending on the privacy practices of your employer, and help an attacker to link your addresses, which otherwise, would be completely isolated.