You were probably working from the same computer with the same IP address and the previous cookies for your other FB account. I don't know how much they track but knowing the same member computer is creating a new FB account is trivial to track if you don't start with a fresh slate of cookies.
That's why I use separate browsers for my various FB and Twitter accounts so they can't be accidentally cross associated in any way shape or form. Firefox is my primary, then I use MSIE and Chrome for two others, and Opera for another rarely used account, etc.
If I wanted to be even stealthier I'd set up each browser with a proxy IP or use a TOR proxy to guarantee no chance of cross contamination but so far the individual browser trick seems to work well enough on it's own.