handle lock happens when you make a sudden stop while the handle is turned almost completely to either side. big bikes weigh more to the front because of the engine placement and when the bike stops suddenly, gravity tends to pull the bike to the ground in the direction where the handle is turned.

it can easily happen if you are at a T-junction, you slowly move forward then suddenly got incoming vehicle from the side that you pull the brake by reaction. because you are not ready to stop, there is a chance for the bike to completely fall over if you are not quick enough to put your leg to the ground. sometimes if the bike is too heavy and it is already leaning over, its probably too late to avoid the fall. you'll risk injuries if you try to hold the bike, better to let it go and hope for minor repair

correct me if i'm wrong.