I could swear that in my younger days people used “Que pasa?” as an alternative for “What’s happening?” “How you doin’?” or just plain “Hey!”
But now it’s all about “Que paso?”, often with the last syllable drawn out for emphasis. When I’ve asked Spanish-speaking acquaintances about it, they look at me as if I’ve just suggested putting ketchup on a taco.
So, did it change? When? Why? Somebody tell me what’s happening!
¿Que pasa? is Spanish for “What’s happening?" (Spanish uses the simple present where we tend to use the present progressive; the literal word-for-word translation would be “What happens?”, a highly improbable though syntactically sound English construction.)
¿Que pasó?, on the other hand, is preterite, and basically means “What happened?”, or, in casual usage, “What’s been going on?” (which would technically call for the imperfect or present perfect, or even present perfect progressive, but the simple preterite works in casual use).
In today’s Mexican speech, I would hear “que pasó” more often than “que pasa,” although I have the same memory as the OP. “Que pasó” (instead of “que pasaba” as Polycarp mentions) is more common. For truly present occasions, I’d hear “que honda” instead of “que pasa” these days.
*¿Qué pasó?, *as you mentioned, is a Mexican expression.
In Perú the equivalent is “Habla” (Speak).
I know about que honda, but I’ve often wondered what it literally means. The quick lookup gave me “What slingshot?” which makes no sense.
Try the no “h” onda. 
“Onda” = wave… as in soundwaves, vibrations. Lots of idiomatic expressions involving that word (having good vibrations, being in tune with someone, being in the know, etc.).
So it’s more a “what’s up?” equivalent.
¿Que pasa?
La misma cagada 
I just asked a Hispanic, Spanish-speaking coworker, and she said the normal greeting would be ¿Que pasa?. She said ¿Qué pasó? would be used if you came upon a body in the street.
Thus the humor sketch where a “language expert” tries to define “onda” by showing pictures to a teenager
1st picture, a swimming pool : teenager : “Esta pileta es honda!” (this pool is deep) (the H in honda is silent in spanish)
2nd picture, a police patrol car: teenager : “uhhhh, mala onda!” (uhh, bad vibes!, uncool)
3rd picture, a lot of motorcycles of diferent brands: teenager: “Cual es la Honda?” (which one is the Honda? / Whats the vibe here)
4th picture, the opening scenes from “Reservoir Dogs”: Teenager: “esto es una onda Tarantino” (This has a Tarantino vibe/mood).
So, thats the onda with the word “onda”.
So what’s the infinitive? To my English ears “que paso” sorta sounds like “was hat passiert” in German, related to English “to pass.”
Same root?
Pasar
“What’s shaking?”
Thanks for the responses. I’ve learned a little Spanish, and although I still don’t understand when or why it changed, apparently it doesn’t happen only in my head (hi, Balthisar!).
"¿Qué es temblor? "
For the same reason any vernacular changes… I rarely hear “23-Skidoo, Butch. See ya in the funny papers-- smok, smok.”
Or “Heyyy, what it be, Jack-sohn, What is Happ-a-NIN, muh may-ahn?”
Hmm. Maybe it’s time to resurrect some of these.
(THEN they’ll remember me from the Corporate Sub-Committee Paradigm Shift Meeting).
¿Qué tal?
Is Hey, Baby, Qué Pasó? really the San Antonio National Anthem?
Doug Sahm said so & he was The State Musician of Texas.
Shouldn’t that be “Was ist passiert?”
Where is she from? My girlfriend is from Guatemala and she says “qué pasó” for “what’s up?” even though it doesn’t make sense to me, and actually sort of bugs me because I think of that in my head as “what happened?” and I always want to respond “nothing happened”.
¿Qué tal? Pero, ¡espere, hay mas!" --Guillermo Mays