A bad programming implementation of equipement that adds to mana (and probably health and stamina as well, I didn't check). Example in the beginning of the game you find a novice hood that adds 30 mana. Maybe they did it that way for aesthetic reasons but it adds 30 mana to the total mana amount but also to the current mana. So a player that equips it immediately has 130 mana. It takes the same animation of recharging as when leveling-up (very fast but not instant) . Normally it should have added only to max mana and recharge naturally over time.

The problem is that when the mana is depleted, you can unequip the hood, reequip it to have again 30 mana. Enough to cast again many spells. It becomes more evident with stronger artifacts as specialist hoods, gloves, rings and necklaces enchanted by the player or found. Just have to put them in favorites (easier) and push 2 times the shortcut key. Destruction mages with impact know what that means . Stagger dragon, 1, 1, cast again, stagger again, etc, ...Well, there are many possibilities.

It should be easy to fix, maybe a modder or unofficial skyrim patch? It's a common programming error in computer games it seems (was also there in Disciples and heroes 6 before they patched it).