"flow control" is the sequencing of aircraft allocated by ATC to try to avoid congestion and to (try to) maintain a smooth flow of arriving and departing aircraft.
Aircraft operating into high traffic density airports such as many capital cities eg. Sydney, are often allocated a slot time, ie. a take-off time based on the flight planned time to give an arrival time at the destination that SHOULD involve minimum delay.
Hence, in this instance, had
VB been operating on a slot time (which may also be based on the number of aircraft in certain airspace at the same time) and arrived at the take-off point a couple of minutes early, ATC may have been witholding T/O clearance.
It's also possible that because of OTHER aircraft interrupting the flow,
VB was again held (through no fault of THEIRS) to allow ATC to sort things out.
Hope that helps.