MouseLeave puzzle

[Jim Hurley]

I have a field script:

on mouseUP

   if there is no graphic "arrow" then
     set the style of the templateGraphic to "line"
     set the endarrow of the templateGraphic to true
     set the lineSize of the templateGraphic  to  2
     create graphic "arrow"
   end if

   put the mouseLoc into line 1 of tGraphicPoints
   put the clicktext into whatText

  switch whatText
   case "objective lens"
     put 869,217 into line 2 of tGraphicPoints
     break
   case "ocular lens"
     put 545,263 into line 2 of tGraphicPoints
     break
   case "focal point"
     put 435,327 into line 2 of tGraphicPoints
     break
   default
     put "" into line 2 of tGraphicPoints
     break
   end switch

   set the points of graphic "arrow" to tGraphicPoints --Draw the arrow
   wait 1 sec
end mouseUp

on mouseLeave
   set the points of graphic "arrow" to ""
end mouseLeave

The puzzle is that the arrow shows for 1 second and then disappears, 
even when the mouse is never moved away from the clickText, much less 
out of the field. (The word pairs are linked.)

Why is the "mouseLeave" message being send?

Jim