Usually a 554 bounce code means that the tranasaction has failed for an unspecified reason. See this discussion: Email Suspended Cause: 554 Denied [IPR] (Mode: normal)
There is also a good Bounce Code FAQ from Pardot that explains the codes in more detail and includes common bounce codes for specific email domains.
Like Val suggested, there isn't always a clear reason. If there are high bounce rates, it might be that the receiving server turned off acceptance from your email domain because you sent them too many emails at once.
Which domain(s) were involved? Were there a lot of people from the same domain or company?