>As we expected, Shouji was asked about when the manga would return from hiatus. He answered that he is busy right now working on Triage X and doing research for future Highschool of the Dead chapters. This research involves studying weapons, vehicles, and other things so he can draw with the high level of technical detail for which he is known. He asks that fans be patient and give him time to bring us the same high quality we have come to expect.
This answer seems to put the long delay on him rather than Daisuke Sato. To me, it sounds like maybe Fujimi Shobo wanted Triage X to be monthly to boost its popularity, they chose to put HOTD on hiatus because it would be too much work for Shouji, and now he is doing the preliminary work for chapters Daisuke already has planned out so it will be less work when HOTD returns and Triage X doesn't then have to go on hiatus itself. That's just speculation, though it does sound more plausible than other theories I've had in the past. In any case, things are looking very hopeful now that we know work is being done on the series.