I'm pretty sure the subject didn't move, the shutter speed was ok @1/80s. I was using Canon 70D + 18-55mm IS STM and a built-in flash and shot in RAW. What it could be? What may be causing this?
It is either subject motion or camera motion. 1/80 second is pretty slow for human subjects not making a specific effort to be still. It is also pretty slow to use hand held if you are going to pixel peep at 100% magnification.
The flash is much shorter in duration than the shutter, so the motion that is only illuminated by the ambient light is much dimmer than the light that came from the flash and froze the motion.
For more about how Av exposure mode with automatic flash exposure (TTL) works in many cameras including pretty much all Canon SLRs, please see this answer to How do I get my Canon 60D to use short shutter speeds with flash in Av mode? as well as this answer to Why is flash TTL metering independent from ambient light metering?
The way Canon (as well as many other) cameras behave when using TTL flash with Av exposure mode is called slow speed sync or dragging the shutter. For more on slow speed sync please see this answer to What is "Dragging the Shutter"?
For more on how Canon's TTL automatic flash exposure systems works, please see this answer to How do TTL flash metering systems calculate how much power is needed?