Half of wow and flutter. The tape speed wasn't perfectly constant, so the pitch wobbled.
Wow was slow speed variations (under 4Hz). Flutter was quicker warbles.
Both were caused mechanically. The rubber wheel - pinch roller - that moved the tape might not be perfectly circular, the motor that pulled the reels might be uneven, and the tape case sometimes pressed on the tape reels causing variable friction.
Analog media really were a bit crap. I get the creative distortion idea, but even so - ewww. I'm so happy these kinds of imperfections are optional now, and not baked into music distribution systems as an expected flaw.
Off topic, but only a couple of years ago the 3d printer community re-discovered these same kinds of flaws in the extruder gears that result in visible ripples in prints. When you put a gear on a stepper motor, and set it with a set screw, you’re pretty much always off center slightly.
I wouldn't call the pro reel-to-reels crap. The imperfections were barely noticeable. Besides, there were other pros to the analogue days, like records having their actual own sound with human imperfections in-tact (unless you had a tonne of money and could afford to spend hours slicing tape). I despise how perfect and same-y everything sounds over the past 20 years. Of course I understand that this is all very subjective, but there just isn't as much access to more raw sounding records as, even very talented musicians who are probably capable of getting great takes which still quantize everything.
But ya ya, old man yells at clouds, I know.
I still record to tape, it's a lot of fun (cassette tape)! There are a bunch of problems to solve that take away from "just being creative" but I don't mind it. Part of that is that it's a forcing factor to improve musicianship because punching in on those consumer four-track machines doesn't really work (even though they have the capability).
> Analog media really were a bit crap. I get the creative distortion idea, but even so - ewww.
On digital music production some randomisation (sometimes called swing in sequencers) is often added so the notes aren't played exactly at the right time.
Perfection often sounds a bit dull the same way seeing a perfectly symmetrical human face feels weird.