The OP describes bees that were being very defensive, actually pestering people who were in spaces adjacent to the hives, as well as defensive when inspecting. Then they became calmer (meaning, not defensive to people in adjacent yard? That's the ultimate test for me to commit regicide). Other local beekeepers have noticed an increase in defensiveness. Couple thoughts on this from things I've seen and heard of:

1. I've encountered defensive bees in a club apiary that were both a pain upon inspection (I'm gentle, avoid crushing bees because any crushed bee could have been the queen IMHO, and I'm at the point I don't need to see every frame to assess the hive, and I use smoke effectively. And my bees were not defensive like this), and were also buzzing the staff at the location 40 feet away who were out for a smoke in their flight-line. 

In that situation, they were not always defensive. Others inspected the club hives and found them to be acceptable (though they were more willing to put up with rude bee behavior). It was during spring as well. These bee-otches were requeened despite the intermittent defensiveness - after removing the queens and a week or so later popping all the queen cells from frames shaken free of bees to be sure not to miss any queen cells. These queenless hives killed the introduced queens after they had laid for a short time, and... after about a few months, those on-location-mated queens were heading defensive hives again. Since that area had a reputation for hot hives for several years, we guessed there were defensive drones around...

2. A beek posted on Facebook in my area of a situation with a hot hive (in his customer's hands, from his nuc originally tho) that he moved and then was not hot after the move. Something about the process of moving? The most defensive bees did not re-orient to new location and were lost? Stress in moving leading to changes in defensive gene expression? A bee mystery. It's an n of 1, but carefully documented. Would not explain other local beekeepers seeing defensiveness as well.

3. The flight-line can change as vegetation grows or is cleared, leading to either a tighter approach or new avenues for by foragers approach to the entrance. I have one hive in my apiary which had a tight space to take off, only a couple feet from hive entrance to a short fence that must be cleared, and they seem to fly lower on approach and have bothered us when in their flight line 20 or so feet away. 

4. In mid March, us northerners will often have a disproportionate number of older bees to younger bees - the overwintered bees with an attitude are raising their sisters. This makes working a hive in that state more likely to trigger defensive behavior. But this should pass once the hive has doubled in population, for me in NE OH has taken until after April 10 really - spring was a slow start this year. Maybe the stars aligned in the OP's area too for a glut of older bees in late March?

5. If last year a few hives were acquired that had very defensive genes, OR had genes very different from the background genetics, like pure Russians (the F1 crosses are, well, cross, is what I have been told), then those hives would have produced drones with defensive genes. Last year, those drones would have found your area's virgin queens, and this year they would be defensive. If one gets a package, very likely the queens were superseded that season, so last year's new packages could have raised queens same year who mated with those drones. 

It's my nightmare because likely the defensive gene has "contaminated" the area. People throw swarms, thus potentially creating a reservoir of defensive genes that can persist. People requeen their managed hive, but the defensive drones from before the requeening live on and continue the "infection" of new queens in the area for awhile. Sometimes also people give up on a defensive hive, also allowing defensive drones to continue the "infection" of the area....

If the area now has a plague of defensive genes carried by the drones, then in the short term, one would want to stick to purchased queens, and avoid raising their own, or find an area with "nice boys" to raise queens in, an outyard or another apiary. In the long term, one would  want to allow the hives with nice queens to make a serious surplus of drones (like, 3 drone frames in a double deep), to hopefully allow nice genes to out-compete the defensive ones. That's a bigger investment of effort than just requeening...

Keep us posted... always helpful to hear other's experiences with defensive hives and the outcome...

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