When someone gets banned for, for example, hacking, they run a check if they have any alternate accounts. If they have more than 10 or so, they "assume" they are public alts, because who needs 10 Minecraft accounts? So your account was on of those 10+ accounts.
I don't know if you did any of these things, but here's a few things they do to crack your account:
-"Free ranks": you fill in your password and e-mail and they tell you you'll get a rank, it's actually a scam.
-"Free Optifine cape": same tactic, they tell you you'll get a free Minecraft cape, they now have your password and e-mail.
Also, if the password of your e-mail is the same of your Minecraft password, they are able to sell it as "full access Minecraft alt"! So make sure those are different.
@CyanYellow lock?