Alloy wheels and flex?

[dddd]

I'll second (third?) the recommendation to first verify uniform high spoke tensions.

As noted, thinner and fewer spokes of course allow more flex.

But what of the rim itself?
A rim gets stressed laterally where the tire meets the road, and of course a flexier rim will allow more flex between the hub and the tire's contact patch.
But, a stiffer rim will more effectively mirror an (albeit reduced) amount of lateral flex up from the roadway, to the brake pads.
The flex between the hub and the roadway is reduced, because the spoke stresses are less localized to the "lower" spokes, but again, the lateral movement down there is also more effectively transmitted upward through the stiffer rim, pivoting about the hub.

Theoretical considerations (that I hadn't yet considered) aside, I was actually surprised by this effect after changing one bike's 40h single-walled rim to a very sturdy touring rim having double-wall construction, then began experiencing brake rub that required a looser brake adjustment.