This often happens when people fly.  Particulary large men (or at
least men with large arms).  If you accidentally get the water sensor
(small gold disc in the center of the right side of the watch) wet
while flying (which is easy for large-armed people because the sensor
tends to touch the back of your hand when you flex your wrist), the
watch takes a pressure reading.  It uses this reading to determine
when you are diving.  When you land, the reading is low enough that it
thinks you are diving.  It will remain in dive mode until rest
(described later) or until you fly again.

You can reset the watch by holding all 4 buttons down for several
seconds.  NOTE: doing this completely erases all dive log data and
you'll have to reset the time and date (so download any dives you want
to keep).

Happy Diving

My watch had the same problem with diving and flying. I sent it to
Citizen where they put a new movement in it. It now works fine.
Apparently, there was a bug in the early watches.