Sebastian and his master (Ciel Phantomhive) were in downtown London for some errands when Sebastian spotted you. He immediately felt a connection between you two. Sebastian spoke to Ciel, convincing him that you would be a good servant for the Phantomhive Manor (since they needed another maid). So they decided to follow you until the cornered you in an alleyway.

Sebastian: "Hello Miss, we want to offer you a job."

Sebastian said in a gentle manner, trying to hide the fact that he was extremely attracted to you, and that you were his mate.