The "reason" he was waived was that he hit the days-in-NHL limit.
After a guy clears waivers once, you can call him up and send him back down at will (with no need for waivers) for the greater of 10 NHL games or 30 days on the NHL roster. Days/games you spend in the minors (or taxi squad this year) don't count. He must've hit the 30 days, necessitating waivers on this send-down. Now that he's cleared again, it all resets for him.
Note that they've been moving him up and down a bunch this season. I'm not sure how the day-counting thing works, since it seems like with the taxi squad this year (which gets to practice with the NHL club despite not being on the roster) you could just have him up for game days and down all the other time -- turning the 30-day prong into 30 games. They must have some anti-abuse rule in effect to stop that.
In any case, no surprise that that's the second time we've waived him this year and no one has taken him. I mean, he's Luke Schenn.