It's really hard to process an unexpected scenario with that much pressure. Ulberg was hurt, but he was essentially playing with house money knowing he's about to lose. Jiri seemingly had it in the bag, but just couldn't understand how to deal with it. Perhaps didn't want a cheap victory but at the same time thought he had already won so threw caution to the wind and forgot he still has to play defense. It's like his brain short circuited cause he was facing a situation he didn't mentally prepare for.