Why did New Hampshire shift so much to the left ?

[prestructuralist]

Are you asking why the State shifted so much to the left when it
1) reelected its GOP governor in a landslide
2) flipped both its houses to the GOP ?

I'm still confused as to why #2 happened.

Moderate Suburbanites in the southern part of the state voted for GOP candidates on the state level as a "Check" on Joe Biden. Many of these suburbanites are Fiscally Conservative, Socially Liberal, and wanted Biden to appoint Liberals to SCOTUS while wanting the State Government to cut their taxes.

Worth remembering also that the GOP only won the popular vote for the State House by 2 percentage points and for the State Senate by ~3.5 percentage points. It's easy to frame this as some huge wave, but really they just won a ton of seats by a very narrow margin. Ticket splitting was certainly a dynamic here as you state (unclear whether actually 'check on Biden' or more just Sununu coattails), but so was gerrymandering and the element of chance conducive to wild swings associated with having so many tiny districts.

I also feel like with Trump's turnout there were likely a bunch of small districts that were flipped simply due to a bit more Republicans coming out than in 2018.