To clarify this, it's not a change in voting patterns but in the Ulster Unionists' alliance with the Conservative party.

If you go to p.23 of this House of Commons briefing paper you can see election results for Northern Ireland. Here's a cut out version of the relevant table. If you look there, you'll see a sudden drop in Conservative MPs, replaced by Ulster Unionists.

However if you read the notes they point out

Ulster Unionists took the Conservative whip at Westminster until 1972 and are listed as Conservatives for general elections up to 1970; from 1974 they are listed as UUP.

So in fact, these "Conservative" MPs were always Ulster Unionists, but could be counted as Conservatives for many purposes, eg. for the crucial fact of counting a voting majority in Parliament. Nothing happened to voting patterns in Northern Ireland, but the Ulster Unionists draw back from the Conservative Party. The reasons for this are explained by /u/redditho24602.

You can also see the origin of two of today's main parties, the SDLP and DUP in the '74 election, starting to squeeze out other small parties. Roughly speaking this was prompted by the same forces, with the SDLP taking a more moderate stance than the UUP and being less sectarian, while the DUP was more hardline.