In every kind of hiking/walking outdoor activity, at some point you have to pass through some chokepoints where it is unavoidable to be very close to other people.
While passing upcoming people, one would breath straight into the others face (and maybe breathing hard).
Thats the perfect nightmare for transmitting Corona virus.

Plus, you never know who is already infected without symptoms, and who is a high-risk person, or who will have contact to a high-risk person the next hour.
Responsible hikers should wear a mask.

California is going to shut down parks and hiking trails, other states might follow soon.