I don't think the aircraft ever left Toronto. It just got delayed till 5am for some reasons. It looked like a mechanical reason.
It looked like the crews would run out of time to go nonstop to HKG when they were ready. It is possibly easier for everyone to go to Osaka and get a crew change.
Here is the info from expertflyer:
"DOCX829/25FEB
* OPERATIONAL FLIGHT INFO * CX 829 0 MO 25FEB19
CITY INFO HOUR (LOCAL)
YYZ LEFT THE GATE 0426
TOOK OFF 0509
ESTIMATED TIME OF ARRIVAL 0838 KIX
KIX ESTIMATED TIME OF DEPARTURE 1000
ESTIMATED TIME OF ARRIVAL 1305 HKG
HKG AIRCRAFT EXPECTED AT GATE 1315
*1A PLANNED FLIGHT INFO* CX 829 0 MO 25FEB19 ASM
APT ARR DY DEP DY CLASS/MEAL EQP GRND EFT TTL
YYZ 0420 MO JCDIWREYBHK/BD 77W 13:40
MLVSNQO/BD
KIX 0800 TU 0900 TU JCDIWREYBHK/BD 1:00 4:15
HKG 1215 TU 18:55
COMMENTS-
1.YYZ KIX - I/ TECHNICAL LANDING
2.KIX HKG - I/ TECHNICAL LANDING
3.ENTIRE FLT- MEMBER OF ONEWORLD
4.YYZ KIX - DEPARTS TERMINAL 3
5.KIX HKG - DEPARTS TERMINAL 1
6.YYZ KIX - ARRIVES TERMINAL 1
7.KIX HKG - ARRIVES TERMINAL 1
8.ENTIRE FLT- 1/ MOVIE
9.ENTIRE FLT- 10/ SHORT FEATURE VIDEO
10.ENTIRE FLT- 12/ IN-SEAT POWER SOURCE
11.ENTIRE FLT- 15/ IN-SEAT VIDEO PLAYER/LIBRARY
12.ENTIRE FLT- 2/ TELEPHONE
13.ENTIRE FLT- 4/ AUDIO PROGRAMMING
8.ENTIRE FLT- 1/ MOVIE
14.ENTIRE FLT- 7/ DUTY FREE SALES
15.ENTIRE FLT- 9/ NON-SMOKING
16.ENTIRE FLT- SECURED FLIGHT
17.ENTIRE FLT- ET/ ELECTRONIC TKT CANDIDATE
18.ENTIRE FLT- J40W32Y296
19.KIX HKG - CO2/PAX* 183.59 KG ECO, 183.59 KG PRE
(*):SOURCE:ICAO CARBON EMISSIONS CALCULATOR
CONFIGURATION-
77W J 40 W 32 Y 296 "