1. Distraction -- staring down at phone/tablet/device;
2. Air-headedness -- they seem to think they're at the mall, and really don't seem to realize they're about to amble into a lane of speeding multi-ton vehicles;
3. Dementia/schizophrenia/drugs -- they don't know they're on planet earth, let alone near traffic;
4. Probably the most annoying: a stubborn political insistence on "pedestrian rights." You know they type: "I have the right to cross the street at the crosswalk, and cars have to give way to me, so I'm going to insist on that right, and not bother to exercise any caution, and won't even bother to look at oncoming traffic because that would imply I'm not secure in my own rights, so I'll just march out and (*SMASH*) AGHGHCKCK!!!!"
"....Well, the knowledge that I had the right of way makes this full-body cast so much more enjoyable."
YouTube is FULL of these.
The most grim ones comes from Russia, for some reason.
My own experience this very morning: