There are several websites which offer to remind you of your friends' birthdays.
One example is www.birthdayalarm.com.
Of course it only works if the website knows your friends' birthdays in the first place.
So, when you register, you can upload your address book, and the website sends all your friends an email asking them to submit their birthdays.
What has probably happened is that someone called Trevor has your address in his address book (even if you don't remember who he is and haven't had contact with him for the last five years!).
So no, it's probably not a scam, just a long lost aquaintance who'd like to be automatically be reminded of the birthdays of all the random people in his address book .