Leadership from both parties were sponsors. When it came over to the House it was sent to the Executive Committee rather than the Elections committee. So it is pretty clear that the goal was to pass a bunch of noncontroversial (or at least agreed to) election law changes that might get tangled up in procedural problems if placed in separate bills.

The undervote provision was an amendment to existing provisions regarding notification about overvotes. The charitable explanation is that someone saw this as a way to catch inadvertent voter mistakes so that they could be corrected.

The cynical explanation is that someone thought some voters were not robotically completing the ballot after the straight ticket was eliminated, and this would detect defective robots. It is pretty likely that all statewide constitutional offices would have candidates from both parties. To someone who believes that elections are simply between the two major parties, there may be no plausible explanation why someone would vote for governor and not treasurer, other than they didn’t understand how to continue voting “D” or “R” throughout the ballot.