There could be 3 reasons why you need to pay extra while downloading a video.
If you have exhausted your download credits for the month provided in your current plan, you'd need to pay extra for a premium download.
You'd be charged while downloading if you use any Elite Assets (payable stock images & videos) in your videos.
If you have exhausted your monthly premium asset credits, you'd be asked to pay for it accordingly.
You can check your Assets credit count and download credits count following the below:
Account → My Plan → Asset Usage