All that is perfectly reasonable. Could be he figured it was better to take a couple shots from this guy than to hit him and wind up dealing with a ton of other bikers, who knows. Frankly the story is something of an urban legend mostly run out by Howard Stern.
Here's a set of links about it. Just sifting through, you can read he was drunk, he was sucker punched, and any other variety of versions of it. But there's enough out there to believe that something happened.
http://www.google.com/search?hl=en&source=hp&q=chuck+zito+van+damme&gbv= 2&oq=Chuck+Ziton+&gs_l=heirloom-hp.1.2.0i13l10.1125.4297.0.8078.14.10.1.184.108.40.2064. 1642.0j9j0j1.10.0...0.0...1c.1.rZvPB1xSjUs