Law of the Grocery Store: Within 10 seconds of entering an empty aisle, it will fill with other customers blocking your access to the item(s) you want.
I see that one all the time. Here's a related phenomenon.
Just yesterday, I headed to the dairy aisle to get some yogurt. There was only one other person in that aisle...on his phone standing with his cart directly in front of the exact brand and size of the yogurt I wanted. It happens A LOT.