Is it possible that the enemy Fiora and Warwick (and to a lesser extent zed and twitch) were recently shut down?
I'm wondering if even though he's still behind your opponents, they may have already given up larger bounties than he currently has. I.e. in round 1 they gained a large lead, in round 2 yasuo made a partial comeback (to set his own potential bounty, which didn't show because it was smaller than all other Champions) and your opponent's bounties were claimed, so that in round 3 yasuo now gets a bounty?
I'm not claiming it's fair, but it's definitely less unfair to get a bounty on the last place player if the other players in the game that are now only marginally ahead of him have already given large bounties. In fact, in those players shoes it would feel unfair to NOT receive a bounty for killing someone that has caught up to you partially through his allies cashing in on your bounty earlier.
Although personally, I really dislike the new bounty system as a whole. CS bounties are dumb (specifically when they further increase a bounty on kills - I admire the purpose of denying proxy singed from becoming worthless while getting lots of gold himself, but I despise the result that getting kills and earning a cs lead through Lane dominance just leads to excessively high bounties), 1000g bounties are pretty dumb (especially on non-carries).
I've purposely walked into the enemy support at low health and traded with them to get rid of my 1000g bounty on maokai/Galio (when no objectives were available / we wouldn't lose anything on my death). There's little doubt I'll die as a front line champion in a teamfight eventually, and giving that kind of bounty to the enemy's carry who's already doing even / better than my carry is going to be disastrous.