How come sometimes when you walk into a store you get BLASTED with like a jet engine size burst of air?

Question

By the way, I know the answer but I want to see if anyone else knows it. 10 points to whoever gets it right first.

Answer 1

it is to keep the bugs from flying in. Hot air out, And dirt from flying in. If you notice the air is blowing out the door as you walk in.

Answer 2

One of two things:   One and most likely is the air curtain,  it's a device that blows air rapidly accross openings;  it keeps bugs out but also keeps outside air from coming in which would cause their AC system to work overtime to keep the air quality right inside the building.  

or it could be positive presure from the air conditioning system, another nice feature for keeping air quality inside a high traffic building.

Well?  How'd I do?