It was a little scary at the time because I thought it was going to land on me! I think it was in the Fall of '08 at the beginning of the '08-09 season.
I had just shot a Gray and had gone outside to collect it. I knew its partner was up there in the trees somewhere not far away because after I shot the 1st one, I lost sight of the 2nd which was only a few feet away from its buddy. So I am out there holding the dead squirrel and looking up at the trees above me for the 2nd one. A few secs later I hear something coming down from the tree behind me, hitting branches, leaves and all on the way down. I turned my head quickly and when I saw the squirrel falling, it was still about 12ft up and almost on top of me! I had barely enough time to back up 1 step. It was actually going to land very close behind - like a few inches away. This one had to have fallen from at least 30ft up because it took me at least 1 second to react and turn my head around and my body a little. The squirrel landed about 3.5ft from me with a loud thud after I had backed up 1 step, and ran up another tree immediately. That completely took me by surprise and raised my heart rate a little.
I didn't know what to make of it at first. I thought it was an Ninja squirrel attacking me to avenge his buddy's death! hehehe, until I calmed down and realized what had happened. I think he was concentrating on hiding from me at a weird angle or position probably, on a small branch, or looking down at me, that he lost his grip and fell. How else can you explain it? Or it was so afraid, it had a panic attack?

I remembered I had seen them fall before so then I calmed down and realized it wasn't a personal attack
Before this one, I had another one fall off my fence that summer which is only 5ft high. I was target shooting from the usual window. I left for a few minutes to get some more pellets or something and when I returned, there was a squirrel at the bird feeders on top of the fence at that time. I couldn't shoot him - season hadn't started yet, so I figured I keep target shooting. The target was about 10" over from the squirrel and about 12yds further back. I took the shot with the Diana 36 at the target, and DOWN goes the squirrell! At first I thought maybe I had hit him, but there was no way my shot was off by 10" or more! He just got up and ran a few feet, then looked around like "what the hek was that?". I then watched him climb a tree and did not look wounder or anything. He was just startled so much by the loud report of the Springer, that he lost his grip and fell. The Diana 36 (same as 34) is pretty loud, and he was only 14-15yds out, plus the sound bounces back from the trees along the edge of the lawn and on the hill, so it sounds louder than what it really is. My 350 Springers sound almost like rimfire because of those trees bouncing the sound waves back at me. Luckily the fence is chain-link. I would imagine if it were wood with very small gaps, the shots would be louder.
But yeah Dave. Squirrels will fall. I am glad you got to witness it. Not many people do, or pay attention. A hunter's eyes catches a lot more critter activity than most people.