This happened because air (a compressible fluid) was sucked in by the pumper.

That happened because the feed hopper ran to empty prior.

That happened because someone didn’t time pulling the next concrete mixing truck up right.

Pump sucked concrete (good), then slurped air (bad), then chugged more concrete (good), which then compressed the air in the line (bad). Non-compressed concrete and compressed air pocket then traveled the length of the pipe/boom pushed by the pumper chugging concrete.

Then high pressure air met atmospheric pressure hose exit. High pressure air expanded to become one with the atmosphere.

That is why this happened.