Keggs is a background figure in this scene (he's serving dinner) but in reality he has orchestrated a good deal of what is occurring. With a vested (financial) interest in the outcome of Maud's romance, he is simultaneously conspiring with both Percy and George while serving his own ends. A cool and calculating man, is Keggs... but even he doesn't expect Lord Marshmoreton to cause a major upheaval by taking the bit in his teeth and announcing Maud's and George's engagement.
While Lord Marshmoreton's precipitous declaration is mistaken and complicates life for both Maud and George, we can but be happy that he found the courage to make it. Always before in conflicts with Caroline or Percy, Lord M. has given in to their demands or done what they wanted because he wants a quiet, peaceful existence. We've seen him changing through his association with Billie and, to a lesser extent, George, becoming more willing to stand up for himself and take independent action. His championing of Maud's cause may be misguided but it's truly done out of his desire to see his daughter happily married to a man of her own choosing. Of course, we find out what has inspired Lord M. to take a stand during his confrontation with Caroline and Percy in the library: he himself has married someone- Billie- whom his family will consider wildly unsuitable- much more unsuitable, in fact, than Maud's supposed choice of George. It's a delight to see him get the better of his sister and -especially- Percy, and have him end up with goodhearted, down-to-earth Billie.