A power outage explains the delay in departure and the rolling starts
The departure procedure is suspended, as well as the choice of rolling starts, explained the FIA, due to a power outage in the starting systems.

A few minutes before the start of the warm-up lap, the FIA announces that the starting procedure is “suspended.” A message that is rarely seen and everyone believes that the weather conditions are the reason for this delay. A delay that many find questionable, considering that the track still appears to be usable at this time.
But the FIA communicated the real reason after the race and it wasn’t the rain that disrupted the starting procedure but a much rarer event: a power supply issue with the starting tools, including the five lights. (e)
The delay in resuming the race after the heavy downpour was due to an electrical power supply problem with the starting systems. (e)
Then, everything could be restored for the actual start of the race, but as a precautionary measure, the FIA decided to launch the Grand Prix behind the safety car.
This also partly explains the fast restart after the red flag caused by Mick Schumacher’s accident. The decision to resume the race after the red flag period behind the safety car was mainly due to the fear that the starting systems would not work properly and the fact that the grip level on the grid was uneven due to the drying track, the FIA stated.
This malfunction partly explains the delay of the start of the Grand Prix, as well as the decision not to have standing starts throughout the race. However, despite everything, it seems that the rain also played a role, as everyone thought during the race.