honestly, i was very shocked at first as i was reading this chapter. i was even thinking "man, this is harsh even for Byakuya... whats up with him?"
Tite was doing it to make us think "whats gotten all them so riled up against Ichigo?". its quite clever, it makes us worry about Ichigo, think of possibilities and thus become more involved in the manga. i thought of many possibilities, the main two were:-
1) Because he is part hollow. as far as i know in the canon, Kenpachi doesn't know about Ichigo and a Hollow. Byakuya did know as he witnessed the hollow's power, but he saw Ichigo fighting against it, whilst this time, they both saw Ichigo co-operating with his inner hollow in order to fight.
i thought that they were all entirely disgusted by this, and were disassociating themselves from him entirely. This theory was almost solidified by Mayuri referring to him as "half shinigami"
2) That the captains had actually surpassed Ichigo. This actually came from the chapter before-hand, when Ichigo's full strength getsuga tenshou only nicks Yammy's neck, then suddenly Byakuya comes along and lops off a leg in his sealed state. This still confuses me, how the hell did they do so much when they should be weaker then him.
But really, the captains were trying to make Ichigo sulk away from them and bog off to Karakura town. this would be easier if they bullied him so he didn't want to hang around them anymore. this makes sense, and its not the first time things like this happen in manga's.
what still isn't explained is how Byakuya lopped off Yammy's leg in a sealed state

sigh...