I've noticed and i think it's the software PCA has installed to turn words into links.
In the text at the top "Do you have a bee in your bonnet about something loosely connected with computers or computing, or anything that makes the digital world tick?" after a second or two "computers" and "computing" turn green bold and highlight into links. I believe it's this action that causes the "jump". The length of the text in the Speakers Corner means 3 lines of text suddenly becomes 4. The other forums don't suffer this.