Suspended from parachute connected to cord holding the box shut. Once parachute no longer pulling on cord (on ground), box easily can be pushed open.

Beavers in wooden boxes, what could possibly go wrong!

In fact, dozens of them on an airplane, this should be turned into a prequel of the Airport film franchise: "Airport '50: Beavers on a Plane"

On a more serious note, the ropes keeping the boxes closed seem to be tensioned by the chutes, so the boxes were probably expected to open on impact. Note how the rope also passes through the inside of the box, so they may have hoped for a fallback via the mechanism of a terrified, trapped beaver gnawing on stuff for stress relief. I suspect that most of people involved knew more about the intricacies of beaver containment than about those of airdrops, so I'm more retrospectively concerned about the beavers' safe landing than about their subsequent escape from the return capsule.

Perhaps by the cameraman that filmed them leaving the box?