Tom Trbojevic’s hamstring strength tested excellently just days before this latest setback. Included Nordbord (see video below) which measures hamstring strength & side-to-side differences; helps to identify players potentially at risk of hamstring injury:
https://www.dailytelegraph.com.au/sport/nrl/teams/sea-eagles/nrl-2020-manly-star-tom-trbojevic-opens-up-on-hamstring-injury-results/news-story/e1d87468f3e5867e685d2ea4494daadc

Lots of “what’s he done wrong/what’s the cause”, need to take into account hamstring strains are the most common injury in the NRL year in & year out (usually ~ double the 2nd most). Hamstring injuries are usually multifactorial & the demands of the game play a significant role

Outside of strength, running biomechanics (the “way” Trbojevic runs) can contribute. See below very similar circumstances in all 3 injuries, running ball back with full stride to step off left foot. Also chance past injuries have caused maladaptions that are contributing