They're probably all much closer together than an elephant length right? So them all turning into elephants would cause the size of the swarm to instantly and violently expand, possibly ejecting some of the outer ones at great speed and endangering everyone within a considerable distance.
So you're saying it's a pressurized pachyderm propulsor? An elephant ejecting explosion? A hephalump hurling horde? I like it, and I definitely like the potential for kinetic chaos.