...it's like peeling a banana halfway and then squeezing it.  The banana will pop out.

I've only been with one civvy who did it involuntarily when having an orgasm.  Providers usually do it on purpose to make you think they came.  They do their Kegels, you know.

It was actually fun - we'd play this game of her trying to expel me and me trying to stay in - only caveat was it's the same muscle group that passes gas she there were times when she'd end up farting - just made both of us laugh

I can think of 2 anatomic reasons or mechanisms that may be in play.
1) Vaginal contractions - these may result involuntarily during arrousal or orgasm.  These muscles may be enhanced by Kagel exercises, but Kagel exercises were designed to strengthen pudental muscles inorder to reduce incontenence.  In teaching Kagel exercises one must contract the muscles that are usually used to pinch off a loaf or hold in the urine or stop urine flow.  Contracting Kagle muscles just contracts the outer muscles, not the entire vaginal muscles.

2) Val salva maneuvor - this results from increased abdomenal pressure, bearing down like you are going to have a bowel movement.  This usually is voluntary, although it can be quite involuntary during orgasm.  During val-salva there is direct pressure to expel all contents from the vagina, like having a bowel movement, or having a baby.  For those who have given birth, the connective tissue that suspend the vagina in the pelvis and keep it from prolapsing or turning wrong side out through the introitus, these muscles, connective tissues, and ligaments often get stretched, lax and in extreme cases, women are left with a pelvic floor that is week and the vagina, uterus, bladder may prolapse a little or a lot with cough, sneeze or val-salva from any cause.  In extreme cases they may be incontinent of urine or have frequent episodes of vaginitis because the vagina is wrong side out and exposed to dry air.  The treatment is vaginal or bladder suspension surgery and a variety of surgical proceedures have been invented for women with severe pelvic relaxation.  Its one of the most common urologic problems from women of middle age.  This pelvic relaxation often causes the penis to be squeezed or ejected from the vagina when a woman holds her breath, cough, sneeze, strains or bears down as we all often do during sex.