I'd hazard a guess that UO deliberately selected guests who paid for their seat.
If UO selected guests who were randomly allocated seats, being as cheap and stingy as they are, they probably would be worried that that would send a message that by not paying for a seat guests would have a chance of being 'upgraded' to an exit row, hampering guests' incentive to pay for seats in advance.

Management at UO would probably feel that the guests in 25A/B were being 'upgraded' from normal seats to extra legroom exit row seats, but unfortunately this time UO got the wrong end of the stick - those guests paid for 25A/B and really wanted 25A/B instead of this 'upgrade'.

Of course this incident is fueled by the extremely poor handling of UO staff, their blatant & immoral lying, and its already-tarnished reputation for treating guests like express-delivery packages.
Refer to:
- (CNN, Jul 2024) "Airline (UO) apologizes after two blind passengers removed from a plane":
https://edition.cnn.com/2024/07/09/t...hnk/index.html
- (The Standard, Jun 2024) "Outrage as HK Express crew confiscates and eats passengers' food"
https://www.thestandard.com.hk/break...gers'-food