That's pretty much it in a nutshell. Manifolds get bent when a set of doubles gets dropped. Even though it's not moving all that fast, even one half of a set of doubles has a lot of kinetic energy, and when the tank that hits first stops the other will keep right on going. The harder the surface the first tank strikes, the more sudden the stop and the more likely you are to bend the manifold.
The only thing that prevents the second tank from moving is the tank bands. With the old narrow bands with no center reinforcing plate, you had a combination of skinny band with little surface area and you had bands that were both hard to get sufficiently tight and prone to letting the tanks twist when dropped. In contrast, with a modern set of 2" or better yet 3" wide bands with center plate reinforcement there is a lot more area in contact with the tank and they are much easier to get sufficiently tight, creating significantly more friction to keep the tanks from moving.
There are limits of course, but with modern bands, properly installed, a set of doubles will survive a more significant impact without moving than with older bands.
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Of course, it doesn't matter what bands you are using if you a) don't keep them snug, and b) you throw them around. As a case in point, I bought a set of double 95s from someone on Scubaboard about 10-12 years ago and the seller shipped them to me. I expected 3 boxes - 2 each with a tank inside and 1 with the manifold. I received one large, heavy box and knew before I opened it that the manifold would be bent.
When I opened it, it was even worse than expected as the seller had decided that it would be better if he left the bands slightly loose. That manifold was bent nearly an inch out of line. I called him, and then sent pictures so he could submit a claim of damages and never heard from him again - probably because the shipper regarded him as being as big an idiot as I did and refused to pay the insurance claim.
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