Before it found it's way into golf, 'par' was used by Stock Exchange traders to represent a notional 'normal' value for shares. If a particular stock was 'below par' that meant that it was performing badly, which is where the usage you refer to comes from.
Later golfers came to use 'par' to represent the 'normal' score for a particular hole, with 'under par' being something to be sought rather than to be avoided, as in the Stock Exchange.
Chris



