1. iPhones units sold stalled for 3 years in a row. This means the company need to seek every year some trick to earn more from each phone. It is not obvious if they'll be able to do for a long time.
2. Top Android devices are improving and Google Pixel 2 is really an incredible device that is making some iOS users switching. I've no data but my nerd friends are doing this a lot, and sometimes what you see in the nerd-sphere in a few years happens in the normal market.
3. They are struggling at competing in the space of the biggest ecosystems around phones. Cloud, computational photography, assistant.