Maybe someone sent spam out and faked (very common) the sender to look like it was sent from your email address. You'd then receive the reply (bounce) if for example, some of the intended recipients have reject rules active or simply don't exist. The full email headers will give more details.
I should add that the reason they're in your spam folder is likely because your email systems spam settings are working and correctly identified the original message (which would have been included in what you received) as spam.
