All the years I have raised monarch (35+) it has been quite common for them to gather in clusters like this. My theory is the same as Gwynne's, that it is so much quicker and easier for them to make their silk buttons where there is already a nice layer of silk already laid down.
At any rate I have never had problems with them being in each others way when they emerge, and the chrysalid are most often hooked pretty securely to the silk and can take any gentle jostling around that an emerging butterfly might make.
At the nature center where we are raising them for a tagging demonstration this coming Saturday, they are lined up along the edges of the raising cage all in a row with barely a half inch between each of them. It will be awesome to see them come out as at least sixty of them went into chrysalis within days of each other. I will let you know how it goes.