It was a deliberate strategy rather than a pattern, although if you're leaving out Ohio in your calculus you're missing a big part of the picture.

So what's going on from about 1874 through 1896 is that the Democrats and Republicans are almost evenly matched nationally, especially in the Electoral College. It is the main reason why turnout is so high - both parties ferociously focus on getting their voters out - and also a good part of why the fights over disenfranchisement, gerrymandering, and state admission get so nasty because it takes very little to swing control. Republicans pull away in 1896 once Mark Hanna successfully runs McKinley and gets enough additional support from population growth in the West to have a near lock on the Electoral College and a massive structural advantage in Congress. Even after that, though, there are still several critical swing states that have been this way for decades: among them, Indiana, Ohio, and New York.

This is because the parties in those states are as equally close as they are nationally. Indiana has a solid Democratic majority in the southern part of the state and a Republican one further north. Ohio has competing machines in Cleveland and Cincinnati that at times get outright violent. New York has Tammany in New York City along with a fairly robust machine in Buffalo for the Democrats offset by Republican strength upstate along with a strong minority in and around the city itself. It is not well known, but a good part of the vigor in Republicans passing the various Enforcement Acts in 1870 has little to do with the South initially; instead, it is fury by the party that Grant loses New York in 1868, which they blame on immigrants (heavily Democratic) and fraud. From Jenkins and Peck's Congress and the Civil Rights Era:

"New York was the Northern focus of the First and Second Enforcement Acts as Republicans worked to guarantee that voting in New York City was “fair.” As well as can be determined, this was largely achieved. Voter registration dropped by nearly 30,000, and dozens were arrested. But the result for the GOP was the same: the Democratic Party’s candidates for governor and for mayor of New York were reelected (with fewer votes), and the partisan distribution of House seats from New York City elections was unchanged."

What happens as a result is not particularly surprising: at their respective conventions - remember, this is in the days where party bosses in control of their delegations fight it out in cigar filled backrooms instead of primaries - there is always strong support for candidates who have a slightly better chance at winning those states. That sometimes comes at the top of the ticket, but at other times comes in the second slot. The other bonus was that Midwestern politicians who survived the brutal partisan battles in their own states to be considered nationally tended to be fairly competent politicians and often underestimated by the East Coast elite.

If you want to read more about this, the best references tend to be on biographies of significant figures within the individual states. I like Shadow of Blooming Grove for Ohio because it really delves down into the Ohio machines and newspapers fighting between the two parties as Harding rose, a lot of the stuff on Teddy Roosevelt as well as work on the Tammany machine captures the situation in New York, and I don't have a great reference for Indiana but the state library has a grad student who looks like they've done good work on some of the conflicts here.