Water passing through a pipe has weight & speed. Stopping (checking) this flow suddenly turns into pressure which has to be absorbed by the pipe and fittings. The sudden shock could burst or damage the water main.
List 7 points to remember when opening a hydrant.
Ensure valve closed after use
Check frost valve works
Ensure pit left clean and dry
If flooded do not remove standpipe until valve closed
Do not remove standpipe where supply fails or not available until valve is closed