Because your email address is attached to one or more records SCA UK holds in Beacon, the third-party system affected by this incident.
That might be because you donated to us, including through JustGiving, which passes donor details on to the charity. It might be because you are a member or a volunteer, because you have come to a meet-up or an event, or because you have been in touch with us at some point.
If it has been a while and you had forgotten about us, that is completely understandable, and it is simply why we still had your details on file.