If Brooke had kept her tush in the chair the kiss would probably never have happened. Brooke wasn’t satisfied with the words of support. Brooke wanted more. Brooke could have got up out of the chair and gone to check on the baby but no she got up out of her chair and started towards Bill. Bill then rose and as Brooke continued to approach he but his arms around her. IMO Brooke and Bill both leaned into the kiss at the same time. That was a kiss or passion not a kiss of comfort.
Why should Brooke have been vulnerable. Ridge had been busy kissing Brooke each time that he came over since they signed the divorce papers. Ridge had been doing nothing but let Brooke know that he wanted things to work out between them. Ridge had even stopped kissing Shauna.
If Shauna had filmed the whole scene I don’t think that Katie would be able to buy the explanation that the kiss just happened.
Ridge had told Quinn that it wasn’t so much about the kiss because he had been kissing Shauna but it was about who Brooke choose to kiss. Ridge was hurt by the fact that Brooke was kissing Bill but I also believe that he was upset because Brooke was once again betraying her sister.