Flank steak came out chewy this time. What's to blame?

I have stir fried flank steak about 5 times now, but this time it came out really chewy. Is this because I may have cooked it longer than usually? Or is it the meat or some other factor?

Here is the procedure I followed:

1. Cut it across the fibers (perpendicular) into 1/4" wide slices.


2. Marinated the pieces for about 10 minutes in a little soy, wine, and aromatics.


3. Cooked on really high heat for 1 minutes, stir fried for another 2 minutes.


4. Took it out. Put it back in and stir fried for another 1-2 minutes.