If you mean ejaculating handsfree from only anal stimulation, it's happened to me twice - given how much I like to be fucked and the 20 odd years I've been getting fucked that makes it super rare lol. Both times it was when I was in my 20s. But that's specific to me, I think it really comes down to how a guy is wired more than anything else. Oftentimes I don't even get fully hard when I'm being fucked because all my concentration is focused on my hole, the pleasure I'm getting from that, and the pleasure I'm giving back to the guy fucking me. But again that's me; I'm not surprised at all that there are other guys who can do it.
If you mean what I usually refer to as an anal orgasm - an orgasm that's prostate-based with no ejaculation (or more specifically where ejaculation is optional/secondary) - that's happened to me a LOT. But that's a very different experience; it's very much an orgasm but it's significantly more intense and longer, and has more of a full body effect. When it happens to me it results in me losing all control; my whole body kind of lights up and starts... well it's not really shaking per se, it's more like I'm vibrating, I moan and make pleasure sounds I can't control (which can be challenging and/or hilarious if we're trying to be discreet lol), and the strength and relentlessness of the spasming from my hole has triggered many guys to bust their nuts while it's happening.
It doesn't happen every time I'm fucked and it's not something I can trigger at will; factors that make it more likely are how long I've been fucked, the position we're in, how big the guy's cock is, and how relaxed I am. It's always impressively intense when it happens though, and guys who are able to do it usually feel pretty fucking proud of themselves LOL. I've got one particular regular fuckbud who treats it like a challenge when we get together (unless its just for a quick pump and dump) where he first tries to give me one, and then second tries to hold off on unloading while my hole is going nuts wrapped around his cock. He rarely gets 2 points LOL.