The HSBC security guard said "Someone shouted at me to get out of the way and then the window shattered on the ground where I had been standing."If they hadn't screamed at me then I probably wouldn't have been so lucky. I was so shaken," added the guard, who spoke on condition of anonymity.
It fell from the top of two "plant floors" - above the offices on the 43rd floor.
A spokesperson for HSBC said: "We are taking this extremely seriously. It is a ridiculous situation to be in and we appreciate how dangerous the accident was."
An HSBC worker inside the tower when the pane fell said: "On my floor we all thought it was a terror attack - we were terrified."
Adjacent windows on the plant floor - which houses are being tested. Every piece of glass has been collected in order to establish the cause of the incident, which happened at around 3pm on Monday (June 9).
It is understood two possible reasons for the incident being probed are a fault in the specific pane of glass or damage caused by maintenance.
An almost identical incident occurred last summer as another pane of glass - on the same side of the tower and just one floor below - fell to the ground in the early hours of the morning.
A Canary Wharf Group spokesperson said: "We will be talking to professional consultants, suppliers and all involved to ascertain as soon as possible the exact cause of this incident.
"The situation was identified very quickly. Both HSBC and Canary Wharf Contractors are working together to identify the cause of the problem."
HSBC ruled out the hot weather as a cause.
The glass was supplied by Guardian Industries Corp. who also supplied windows for the McGraw Hill, Clifford Chance and Lehman Brothers buildings.
An HSBC worker said: "We were sitting at our desks when a couple of people said they had seen something fall down past the window. We went over and saw all the glass smashed on the floor below and couldn't believe it.
"Any of us could have been outside at that moment, either having a cigarette or just getting some fresh air.
"We heard about the one that fell out last year but thought it would just be a one-off.
"Now it seems it could happen at any time and it's very worrying."
Tower Hamlets Health and Safety said: "The incident has been reported, and our inspectors will investigate."