I have had the happen with iCloud and other email providers also. I have an iCloud alias account I have never once used (I just wanted to grab the name) and I got a couple spams sent to it.
I don't believe Apple leaked your email. I think spammers just sling out a bajillion emails to @icloud.com with random or dictionary words and you get hit this way. I reported the message to
spam@icloud.com and they stopped.