Multiple accounts are allowed, but some people don't like it, and may give negative trust for alt-accounts.
I bet they didn't get negative trust just for owning an alt, but what they were using it for.
Examples are:
-quoting your own post with an alt
-praising yourself to make the deal look trustworthy
-giving yourself trust
-post padding by talking to yourself
-spamming other users with negative trust or private messages
And probably many more, people can be really clever