it is b/s ur password got in wrong hand.. either ur pc or other pc have
key logger, which save information what u type by key board and send them to hacker/cracker.
scan ur pc with 3-4 diff. anti virus/spy scanner.
also use on screen keyboard to type password or useful information to save from hackers...
try to scan pc atleast once in a month... use firewall.
also dont write ur password in email or else on internet... avoid fake/spam of yahoo pw cracking msgs...
more information :
http://www.keylogger.org/
if u have suspect, install it on suspect's pc, and write ur email to send u information on some duration...