I understand that Gustav opening the vans causes everything to spin out of control rapidly and even enjoy the implied satire that it is his good, humanitarian side that causes all the trouble. But he opened only three vans and there was a big military presence there. It would have caused some problem but it doesn't seem like it would be that much. The military is shown fleeing right away.
shareI think what probably happened (off-screen) is that the military bugged out immediately when they saw a large contingent of loose zombies, as part of their response protocol. No point taking risks in a chaotic situation...better to fall back to the various roadblocks and take them out from behind barriers safely and effectively as they arrive.
Also likely, as we see at the end of the film that chaos is reigning everywhere: the virus had broken loose from that specific area and they were fighting fires in more and more places simultaneously. So falling back made sense. There was a mention in one of the news reports that the roadblacks were being moved back...
Holland doesn't like Piggy Lookadoo.