Most likely it was a password stuffing attack. If they used the same password on multiple sites, there is a good chance one of those other sites was compromised and the attackers took the compromised credentials and tried them on other sites like Instagram. It could have been something more advanced like a stolen cookie, but usually the simplest explanation is most likely.
Always use a different password for each service, enable MFA where possible, and use a password vault like Bitwarden.