A player enters a slump when he performs far lower than usual for a period of time. Generally after a long period of dominance or a big title win, a player will enter a slump for a term. Sometimes players will only enter slumps for a short period of time; other times it may last many months or even years.
Theories for a slump:
Fame Theory - Stated by Boxer and sAviOr in interviews, as they became the dominant force their priorities changed. Their practice time and drive to win and keep up their dominance faded. Once a player has hit rock bottom and finally sees what they have lost, their will is returned which is why we sometimes see a comeback in severe slumping players.
Age Theory - As a player gets older, their priorities change from being top to adult priorities lacking in the time to practice and will to stay on top..
Confidence Theory - When a player is dominant, he is always confident and performs well. However, after some unexpected losses, the player loses his confidence, starts to doubt his play style and starts to play sloppy.
Rollover Theory - That each new generation of players is better than the last by learning from their mistakes. Just like old people can't adapt to new technologies, some older players cannot or do not have the will to change to a more modern style of play leaving them in a slump. A perfect example of this is when Bisu destroyed sAviOr in a 3-0 Bo5. sAviOr tried to play the way that made him dominant before he lost but the new ways that Bisu presented were better. According to this theory either we have reached a perfection of StarCraft that programers are perfect in every way compared to the older generation or that we are expecting another rollover and dominate players like Jaedong, Bisu, and Fantasy will eventually meet a better generation.
Slower Reactions Theory - As players get older their mental reactions and physical reactions become slower due to aging. Some skeptics say that physical slowing down doesn't take place until later in life because they compare StarCraft to games like Tennis where some players stay dominate longer and further into their life. But the fact that progamers may practice up to 10–15 hours a day of constant finger use does take its toll.
Combined Theory - This theory is a combination of all of them. As a player gets older they start to worry about their future in the e-sport, their younger selves still want to party and have fun instead of practicing, new generations are better but they don't have the will to keep up or physically cant because of the sheer amount of time they used their fingers over the past couple of years.