GoPro has been dying for a few years, there are a ton of alternatives at much lower prices and if you're just strapping it to a bike or something no need to go top of the line.
I'll add that based on their last earnings call it sounded like they learned a lot from xmas 2017. Now they're introducing new models every year and at lower price points. Competition is one reason. Also, saturation is probably why we see that $100 trade-in credit for any old digital camera. There is a slight premium to pay, but in a lot of cases I think it's worth it for waterproofing, build quality, software stability, etc.
Their stock has really tumbled but I think they've figured out the formula now and the stock will probably turn around in 2019 after the xmas sales numbers are in.