The only explanation I can offer for “Slack” is that its founders and top brass have been adherents of the “parody religion” Church of the SubGenius, and have successfully interposed themselves into the everyday business machinations of a wide swath of Western businesses.
Meanwhile, “Discord” wouldn’t surprise me if it were a product of the Discordians themselves, a similarly disorganized “parody religion”.
Slack is also allowance in budgets (time, finance, materials) to allow for unplanned pressures, demands or constraints.
Slack stands for Searchable Log of All Conversations and Knowledge. Prior to that, it was going to be "linefeed".
Also see: Slackware.
I always preferred Discordianism over CofSG, but both were cool.