Why would you think Occam's Razor would not determine that racism is the reason? It's just as simple of an explanation as anxiety.
I'm not sure some people understand how "normal" racism is.
I'm not sure some people understand how "normal" racism is.
Another comment suggested that Altman was once beat up by a black man. If true, it is possible Sam has developed a conditioned response that associates black men with danger and his reaction stemmed from that. However, that isn't the same thing as racism and to try and categorize it as such would be quite disingenuous.