This is a very specific question inspired by the Palin resignation. I can think of three reasons why a governor might resign:
Other than Palin, does anyone know of a US governor resigning when none of these reasons applied?
William Weld resigned as Massachusetts governor in the hope of being confirmed as ambassador to Mexico. He wasn’t.
BTW, we don’t know that Palin’s reason(s) aren’t in your list.
Well, those are certainly the big three reasons. Samuel Black, the governor of the Nebraska territory from 1859-1861, resigned to join the army when the Civil War broke out. There was a territorial governor before him named Mark Izard (1855-57), who resigned, but I can’t find out why. It seems to have been some quarrel with the territorial legislature.
Adelbert Ames of Mississippi might count, but might fit under your category 1. He was Republican governor of Mississippi from 1874-1876, and barely managed to take office in the first place…there were riots when he was elected. In the 1875 election, Democrats took back control of the legislature and tried to impeach him, and he worked out a deal with them where they’d drop the impeachment and he’d resign. So, there was no scandal there, but he did resign to avoid forced removal from office.
Here’s an interesting one. John Dickenson, governor of Delaware from 1781-82, resigned to become governor of Pennsylvania.
A good reference for this stuff is Wikipedia’s lists: “List of Governors of Alabama,” etc. The lists I’ve looked at have footnotes for each governor who left office early. An interesting one: Thomas Stevenson Drew, governor of Arkansas from 1844-1849, who resigned because his pay was too low.
Jim McGreevey (NJ) resigned in 2004 after coming out as gay. I suppose that qualifies as a “severe personal crisis” but so far as I know he was not facing any criminal charges nor an immediate threat of removal from office.
Sam Houston holds the unusual distinction of having been governor of two separate states, and having left both governorships under unusual circumstances. He resigned as governor of Tennessee apparently because of a separation from his wife, which was a “severe personal crisis” but probably wouldn’t be that big of a deal today. Then he became governor of Texas but was supplanted by the state secession convention (technically he neither resigned nor was impeached) because he refused to take an oath in support of the Confederacy.
According to the National Governors Association 205 governors have resigned so it would be a project to track the reasons for all. No doubt the majority moved to federal office.
McGreevey was about to get in legal trouble from his gay lover, see Wiki. IMO, there would have been a metaphorical firestorm if he had stayed in office. I’ll call that Category 1 from the OP.