Showing One Card (after everyone has folded)

[KenProspero]

The other night at Canterbury

Player raises on the river -- everyone folds. Winner then shows one card (an ace) meaning he had paired the board. Another player said -- you have to show both cards, and the dealer agreed. Player showed the second card, and the game went on without incident.

This obviously ISN'T 'show one show all'. The player next to me said he thought it was some kind of anti-taunting rule.

I'd never really thought about it, but is this rule the norm? Is there any other rationale?

The best I can come up with is that so many people misinterpret SOSA, that places make it the rule in this situation to keep the game moving.