I don't know. Some random, complex, interplay of "in the moment" factors would be my best guess.
None of my births have been similar to my mother's births, or her sister's births, or the births of my paternal relatives. All three of my births have been very different although they all happened at roughly the same gestation and the babes were all within a few ounces of each other.
dd1- 42+weeks, 9lbs6oz, and roughly 32 hours of back labor... the final 19 of those "stuck" at 7cm despite non-stop double peaked contractions that nothing could shake. Eventually my water birth ended in the OR with a c/s.
dd2- 40+weeks, 9lbseven, and 12 hours from start to finish. Textbook progress and while there was some drama at the very end, it really was the "birth you read about in the natural birthing books". I used hypnobabies in the birth tub, there were breaks between contractions and those contractions gradually increased in strength/length, etc.
d2- 42weeks to the day, 9lbs8oz, and 4.5 hours from first contraction (with long/hard/posterior cervix) to babe in arms, It was "too fast". It felt like one, long, never ending contraction. I was unable to sit up in the tub, walk, or even roll over on the bed without dh and my doula assisting (I honestly don't remember getting from the tub to the bed and dh said he carried me because I couldn't "uncurl" from the fetal position myself). I screamed more or less non-stop for those 4 hours. The hypnobabies did nothing, I had a shot of stadol that did nothing, and although I felt FINE as soon as ds was out (well, I couldn't speak above a whisper for the entire day after) it was not an experience I ever, ever, EVER want again.
But was there any "real difference" between the births? Not really. I had loving attentive care providers, I had my doula, I had dh... the babes were nearly the same sizes...I did have heplocks for my vbacs and with ds I apparently had a fetal monitor belt towards the end along with that shot of useless stadol (but I have no memory of them putting it on and I couldn't move anyway so no harm, no foul!) but I used a tub in all the labors and always had a lot of freedom to move/scream/hypnobirth/etc... I wasn't afraid of birth and I felt safe each time...
I have to admit I'm really afraid of another "too fast" birth this time round. I think I might actually be more afraid of that than I am of a rc/s. It sounds crazy, but there it is. I'll be working a lot on processing and preparing but I really don't know what choice I'll make if labor hits me like that again.