Hydraulic Cylinder Safety Accident

A technician is repairing the hydraulic cylinder to replace the seal. The cylinder is pressurized to 9000psi/621bar for testing. The pressure remains stable, but the plunger will not retract when trying to reduce the system pressure.
The technician sees that the pressure gauge reading of the pump is zero. Then he disconnected the hose from the pump and connected a connector to drain the cylinder. Two other technicians joined in to try to solve the accident. One uses a wrench to disconnect the nozzle, and the other holds a bucket in front of the cylinder, hoping to catch the oil.
At first, they saw some oil droplets flowing out of the cylinder. At this time, they assumed that the cylinder was not pressurized. On the last thread of the connector, the connector popped out quickly and hit the bucket. The two technicians were knocked to the ground and covered with oil. Fortunately, no serious injuries were caused.
Why did this happen?
If the hose connection is incorrect or the connector is defective, the pressure may enter the cylinder, but cannot be discharged when the pump pressure is released (if the pressure in the cylinder is still blocked, the pressure gauge will display zero).
What should I do?
Seeing the plunger stuck in the extended position clearly indicates that the cylinder may still be pressurized. Connect the bleeding tool to the connector to safely depressurize and retract the plunger.
Important safety points
○ If the hydraulic device supports the load, block and stack the load as required so that the load will not move or fall when the hydraulic pressure is released.
○ Check that the threads on the coupling half to be depressurized are clean and in good condition. Do not attempt to bleed the connector if the thread is damaged.
○ Check that all threads on the bleed tool body and cover/bolt assembly are clean and in good condition. Do not use a bleeding tool if the thread is damaged.
○ Tighten the body of the bleeding tool only by hand.
○ Do not attempt to repair or reuse worn or damaged connectors.