The fact that the boa's head actually slammed into the glass leads me to believe it was a simple instance of cage aggression, which is common in even the friendliest of boas (which is why hook training is such a great thing!). If it was a defensive strike, the boa probably wouldn't have slammed into the glass.
However, the pet store's claim that this is a new and recurring behavior for this snake indicates that it is not cage aggression and is instead a reaction to stress, which is also very likely. Tell the pet store that boas NEED hides--and a cage cover or dark area is never a substitute for a snug hiding place!
If you want the boa, get it. Cage aggression disappears once the snake is out of the cage, and stress-induced defensiveness will also disappear with a few weeks without handling in a peaceful environment with suitable hiding areas. Boas are very sweet snakes, even if hatchlings can be a bit nippy every now and then. Any properly cared-for and handled boa will grow to be a very docile, sweet-tempered snake.
Cage aggression is when a snake displays an aggressive feeding response when no food is present.