It’s a bit. It’s quite popular to link an xkcd whenever it’s relevant, along with one of a few different catch phrases. “Mandatory” being one of those. For this phrase in particular, the joke is that there is usually an xkcd for most situations, and that it is our “duty” as nerds to link to the appropriate one. It’s an inside joke for nerds. I hope this explains the custom to you.

Bobby tables has a gun on my back

> Who made the mandate?

I did

A lot of hassle to save two characters compared to a simple xkcd.com/327 :P