I'm pretty sure Marshall got snatched up by the Red Sox off waivers (late in the season? post-season?), but they waived him, too, which allowed the A's to get him back. Maybe Theo was doing Billy a solid or something.

They've decided that F. Hernandez isn't good enough to stay at the big league level this year...but they'd like to still have him in the organization.

That's where the timing comes in. I think the A's have decided that he has a better chance of passing through waivers now than he does later in the year.

Their logic makes sense for two reasons:
1. It's early in the year, so pitching league-wide hasn't suffered major injury attrition yet (this logic would suggest that a mediocre pitcher is more likely to pass through waivers in April than, say, July, when teams have a few guys transferred to the 60-day DL).
2. Kyle Snyder just passed through waivers, and Yorman Bazardo was just DFA'd. If Snyder passed through it shows that no one team is that desperate for pitching yet, and with Bazardo DFA'd, you could argue that even if one team is desperate, Bazardo's more likely to get snagged, while F Hernandez sneaks through.

In other words, if you flood the market with similar scraps at the exact same time, it's less likely that your scraps get picked up.