One might argue the wording specifically says "...a creature can be targeted only once by each casting of this spell." and interpret that to mean the initially targeted mob upon casting. I'd rule the bounces, if any, can pinball, as long as there's at least two mobs within range to target. Subsequent castings cannot target the same initial mob as a previous casting... Not game-breaking, and keeps the pinball flavor. YMMV 

A level 14 Evocation Mage with the Overchannel ability can cast this spell at level 5 and have it bounce 5 times, dealing maximum damage to each enemy if the attacks land? Or since they don't roll damage dice, but just deal maximum damage, wouldn't it bounce?

Quote from Lirean >>

A level 14 Evocation Mage with the Overchannel ability can cast this spell at level 5 and have it bounce 5 times, dealing maximum damage to each enemy if the attacks land? Or since they don't roll damage dice, but just deal maximum damage, wouldn't it bounce?

A strict reading of the rules might say no, since the bouncing effect is triggered "if you roll the same number". Overchannel says you deal maximum damage, and although it doesn't explicitly say that you don't roll the dice, that's how it's commonly understood. If you're not rolling dice, you can't roll the same number on two dice.

That said, a lot of DMs would probably allow it because it sounds awesome.

Quote from GaiusMarcus >>

My DM just pointed out that 
"If you roll the same number on two or more of the d8s, the orb leaps to a different target of your choice within 30 feet of the target. Make an attack roll against the new target, and make a new damage roll. The orb can’t leap again unless you cast the spell with a level 2+ spell slot."

You get to choose the target, but the spells leaps, that part isn't optional.

 "Of your choice" if you do not choose a target, the spell runs out of vald targets and the bounce fizzles.