phpBB forums do not, by default, allow for html.
You can use the built in bbcode, using things like [b], [url], etc.
You can enable the html through the administration panel on a regular phpBB install, not sure if its the same on a phpNuke module install.
General Admin > Configuration > User and Forum Basic Settings
HTML was enabled, I don't know why it's doing this....I didn't put any html in the message, just plain text...