I've been with a couple women with well-developed Kegel muscles. One in particular could have your sperm in her cervix within 5 seconds after she said, "You're gonna cum now.". Are the anal contractions usually that strong? I know how it feels different (and very nice!) if I cum with a cock inside me, but it's only happened after the top guy inseminated me.
Have you ever shot off while you had slid a finger in your ass? If so, you know that your pucker contracts/clamps down hard as each shot of cum spurts out...If your buddy has his cock in your ass as you cum, your hole is squeezing and releasing on his shaft with each shot...and feeling that happening and knowing you made him cum, it will send you over the edge with him... but not always.