My theory to be confirm with this next jump back to im sols college days.
The taxi driver is sonjae’s stalker and all the attacks on imsol is due to sunjaes interaction with her. And stalker thinking imsol is getting in between him and sunjae.
Original timeline sunjae has crush and starts following after her like a lost puppy, imsol attacked but saved the 2 part ways for 14 years. They meet again and sunjaes makes specifically stoops at the bridge to meet her and receives a gift, sunjae dies that night.
Alt timeline, sunjae still has a crush but imsol now is aware of his presence and they grow closer as friends. She is attacked by taxi driver and saved, again Sujae and imsol go separate ways for 14 years and reunite, sunjae is killed within a day of their reunion.
If parting ways and not meeting sunjae for 14years kept both sides safe until they reunited, with this next time jump to college days and them spending time together again one of them should be attacked again to drive them apart again.