Cookshack says that there are several variables, in my case most probably too much wood smoke at the onset of the cook. At some point, it goes pressure critical, releasing through door and vent, then creates a momentary vacuum before going back to normal. Tech OPS says it's not really an issue, but he heard of incidents on larger commercial models where the "burp" is so powerful that the smoker doors are damaged.
Having never experienced this before, it definitely caught my attention at a distance of 5 feet. I'm glad I wasn't drinking or I might have wondered if this event happened at all.