They picked support Amumu recently too.
Regarding Trundle, I think BotRK synergizes really well with his Q and W, since Q gives 40 extra AD and W increases his attack speed (and self-healing) so you can proc the unique passive from BotRK as much as you can.
BotRK also benefits from the armor shred from Subjugate and the active heal is boosted by W to obscene amounts and also helps you stick to a target (or peel if you have a massively fed carry).
Personally I think Hydra would be fine as a second offensive item but you'd have to ditch Triforce for it (unless you are a man and want to build 3 offensive items).
Edit: Warrick is definitely situational but his W gives like 8000 gold worth of stats to your team which makes him amazing on teams that can siege. Pick him when you have Champions like Caitlyn, Soraka, Nidalee, Jayce etc.
He's also a hilarious duelist.