Can you link it? Was the video streaming or playing from disk? Perhaps it was an unsupported video codec? There are certain circumstances where the pi may struggle to play video, but I think for common formats they graphics driver has hardware level support for decoding. If the video format was weird and it had to decode with the CPU it could see problems. Another issue is the power supply. If you do not use a quality 2A power supply, a little red light by the power jack will indicate reduced power and it will throttle the CPU. This applies to the older PIs, but I've not seen the Pi4 yet.