"This fight is easy!"
D&D battles can become predictable. It seems like a fight to the death would never be humdrum but too often that is exactly what happens. The mid-battle twist is a small effort to make battles feel a little more spontaneous. Enemies should have their own motivations, even monsters. Are they committed to this battle so much that they are willing to die for it? Are there reinforcements on the way? Are there non-combatants swept up in the scuffle?
Maybe that monster killed last round was only pretending to be dead, and is going to stab someone in the back. Or perhaps the terrain is unsteady. You also never know when one of the enemies will pull out that potion they've been saving. Are the combatants even there? Has this all just been an illusion?
There are countless ways to shake things up in your battles. Use this random generator to add those twists and keep your players on their toes.