Refs are not fixing games, but quite (very) possibly gambling interests are buying a couple of calls a game to tilt the outcomes. Not ecery call, and not every official, just as a pennyweight will tip a heavily laden scale.
Wholesale gambling came in under Tagliabue as commissioner. Officiating wasn't perfect, but was better - clear back to Pete Rozelle. Tech improvements chaptered out some of the most egregious calls, that also occurred under Tagliabue as well-meaning coaches on the rules committes did their best to regulate what was slipping out of hand. They had been victimized by it and wanted to correct it. Call it catch-up or dust-up efforts shoveling sand against the tide of enormous amounts of money being bet on games. Today it is what it is.
