Energy randomisation
Speed randomisation is a game mechanic designed to add unpredictability to monster behavior, and to nerf pillar dancing. Monsters normally get to take a number of monster turns per player turn based on their speed: fast monsters act more often, slow act less. However, the game will occasionally and randomly give enemies 90% or 110% of the amount of time they should get, allowing you to occasionally gain a tile of distance between yourself and an equal speed pursuer, or to allow a pursuing enemy to occasionally attack you as you flee.
A monster who gets two actions due to speed randomisation MUST spend one of the two actions on a move.
History
Speed randomisation was first introduced in 0.6. It was modified in 0.7 to ensure that enemies couldn't spend both actions attacking.