The obvious answer would seem to be a live animal, but it doesn't appear that UA transports animals of sufficient size to urinate in that volume. Urine isn't something you normally transport in cargo, except for use as pest deterrent (like coyote urine spray to repel deer). Could be a broken or cracked bottle of such in someone else's luggage? Or a ramp employee expressing frustration? Leakage from plumbing dripping on your bag while in the stopover luggage handling? Any scenario seems odd.