When you switch to Source mode, it looks like the page is getting extraneous javascript events as you type. For example, on enwiki, put this in your common.js:
mw.loader.load('https://tools-static.wmflabs.org/spi-tools-dev/spi/tag-check.js');
maxUsersCUStaleness = 300;
mw.loader.load("/w/index.php?title=User:Writ Keeper/Scripts/cuStaleness.js&action=raw&ctype=text/javascript");
mw.loader.load("/w/index.php?title=User:Writ Keeper/Scripts/sockStaleness.js&action=raw&ctype=text/javascript");
These are three scripts commonly in use by SPI clerks. Now, navigate to an SPI page (say, :en:Wikipedia:Sockpuppet investigations/Jinnifer, reply to a comment, go into Source mode, and start typing. The scripts will run repeatedly every keystroke or so, as seen in the attached screenshot.