The list of valid targets seems to be generated at the start of his cast, however, you don't know what that target is until the cast completes. A target that was at range that ran into melee could have been a valid target when the target selection occured. This happens a lot if there is a vortex near the boss knocking people away causing melee targets to become valid targets of the ability.
Think of it like the empowered shock vortex. If he begins to cast it while you are running to him from switching targets, you likely turn around and run away to spread out. Same logic. If the fireball is already being cast you shouldn't run into melee but be prepared to run away.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.