Reed opening at high mouth pressure amplifies the air column resonance by constructive interference

Evidence

This leaves only one last possibility, case a. This fortunately works well: the mode is mostly air column vibration and the reed opens when the pressure is high in the mouthpiece (resonance enhancement, adding amplitude to amplitude of the same sign). The flow (velocity) does maximum work against the high pressure in the mouthpiece and reinforces the pressure there by constructive interference. — Eric J. Heller, Why You Hear What You Hear- An…, 1613