The root systems on some of these trees are very small, compared to the top growth. I'm wondering what the cause is

Depends on the individual situation. However, if you are talking about ones with circling, kinked or j-roots, minimal rooting out (after having been in place for some years) then those were root-bound due to careless container culture at a production facility - which is a pandemic problem within the nursery industry. And received inadequate or no root correction at planting time, another very common occurrence.

Thanks bboy. It makes me scared to plant a tree without knowing I'm doing it right. Is it maybe a good thing I can't afford the really large sizes? I meant to post the link to the photos I was looking at. http://www.seattlepi.com/local/komo/article/Rare-August-storm-blows-ashore-knocks-out-power-6473481.php#photo-8554672

Above starts off looking like it is trees in compacted turf and other tough situations, where the roots were not doing well enough to hold the trees up. On inhabited sites it is common for trees that have had adversely impacted root zones to be the ones that go over in storms, it turning out that they were sort of perched in position before the wind exposure revealed how tenuously held in place they had been.

In the woods there is a tendency for failures to be caused by the attentions of root pathogens, as well as pathogens that destroy the trunks above the crowns.

A lot of landscapes in newer subdivisions just have a cosmetic coating of screened topsoil on compacted subsoil or hardpan left by the builder's bulldozer doing the final grading.

A tree planted in that environment doesn't have much of a chance of standing up to a strong wind as soon as the tree's 'sail' gets large. They're just sitting on top of the ground, essentially.

I lost a few branches, but no trees.......yet.

Mike