This is from machinedesign.com

All theoretical analyses of self-locking worm gears deal with static conditions. In such an analysis, the load on the worm gear can’t drive the worm if the coefficient of friction between worm gear and worm is larger than the tangent of the worm’s lead angle. In other words, the friction angle must be larger than the lead angle to prevent backdriving.

Within the last 3 years, our failure analysis engineers investigated two accidents involving worm gears where the user was injured. In each case, a designer selected a worm gear speed reducer from a catalog and incorporated it into a lift device intended to raise and lower people. In both cases, the designer was under the impression that the load (person plus platform) could not self-lower. They both relied on self-locking worm gears to hold the cargo in a desired position and genuinely believed that no brake was necessary to supplement the worm gear in such a critical application.

So it is a matter of friction. There is your answer!