My guess is that they realized that some sort of glitch was going on in the system, regarding people's uploaded documents not showing up in the system, etc...
They are probably working around a fix for that before issuing more ITAs and risking having the same issue happening to more and more candidates.
(all speculations though)
Besides, I have seen a very small amount of people actually receiving the PPR, which makes me think that they might want to speed up the current applications to a level that they won't have a huge amount of apps under the initial eligibility review stage. So they can issue more ITAs and keep the whole process moving smooth. But again, that is just my guess :