I think this is part of Howie's strategy to expand the PS as he has done the last couple of years.
He is continually cutting, adding, cutting, readding players - instead of 16(17) developmental players they end up with 20+ players that are getting developmental practice time throughout the year.
Since Hall cleared waivers, no other team wants to add him to the 53. The Eagles may well have discussed it with him and why go to another team's PS, when he can stick around here where they know him and like him and be added back to the PS in a couple of weeks? Ultimately, his chance of sticking on a roster is far greater here in a couple of weeks than on somebody else's who doesn't know him from Adam's PS.
I'd be surprised if he doesn't resurface in a few weeks. The only risk is that some other team adds him to their 53, and the Eagles have already indicated that they are OK with that by exposing him to waivers (which shows them the risk is not as great as some fans may think).