There is also the phenomenon of inertial confinement. The shockwave would travel at the speed of sound times the Mach number of the wave. It could transit a small branch faster than the branch would have time to react. But generally, even small branches should have enough drag to be shaken (at least) by a shockwave.

Or, the explosion wasn't really a detonation, but a deflagration (combustion cloud). Deflagrations produce a spectacle but a much lower pressure signature. Movie explosions are mostly of this kind: big blast of flame. As opposed to the dynamite-the-safe-in-the-mail-car scene from "Butch Cassidy and the Sundance Kid," which was one of the most realistic explosions I have seen on film (still fake, but more like the real thing).