What you were expecting to happen, and what actually happened:
The glowing mana orb is a condition of the Enchant status. It is appearing when any troop has full mana without the accompanying graphics on the troop card.
How often does this happen? When did it begin happening?
Every fight since today?
Steps to make it happen again
Fill mana on any troop.
In 1 pic you can see Leprechaun is at full mana at start. Centuragon has his mana filled in the other pic.
Enchant makes it glow pink-purplish they’ve decided to (for reasons known only to the devs) make it glow blue when that troops spell is ready to cast, it is awefull.
I dont recall seeing this before 7.1, Its pretty obvious. Furthermore Zendesk says this is a condition of Enchant along with the symbols floating on the troop pic. Its a bug and if not they need to change it, its confusing.