Looking at the beginning of the video you can see the clerk looking to his right at someone off screen. You can see he was preparing to defend himself from the 3rd perp who had already jumped the counter to his right in the area of the 2nd perp who was leaning over the counter getting cash.

In other words when the stabbed perp jumped the counter he effectively had the clerk trapped in a corner between two other bad guys with himself blocking the clerks way of escape.

The “knife” looks like a box cutter that you would expect to find in that kind of place which is a tool for opening wholesale boxes and that stupid overkill Chinese plastic packaging that is impossible to open without a blade…and have been banned from airplanes since 2002.