FWIW, my alcoholic ex got like that too - hair-trigger temper, would get very angry seemingly for no reason - and he too used to be mild and laid-back (and certainly not rude to frontline service staff). I think it has to do with either a lowered threshold for impulse control (no more stopping and thinking before going off on someone) or anxiety (something is WRONG in this situation and I can’t control it and it makes me crazy!).
When this happened (before I left him), I found the most useful response was to disagree with him politely but firmly (“I don’t think the price they’re asking is unreasonable”). This is also indicates to the person he’s berating that they are not the problem here.
Ultimately, unfortunately, the only way to not be embarrassed by a volatile alcoholic in public is to not go places with them.