I always blame Russian hackers when GPS dropouts mess up my Strava logs.

But, yeah, it's usually atmospheric conditions, terrain, man made obstacles, etc. In my area usually when anyone experiences GPS glitches, several folks report the same problems.

A couple of years ago Strava wasn't very good at recovering ride logs when glitches occurred, while Cyclemeter did very well when I ran both apps at the same time. Since then it appears Strava is better at guesstimating our likely routes and recreates routes and related data pretty accurately despite GPS glitches.