Cool - A question to which I know the answer. Any animal which migrates does so for one of the following reasons.
1. A lack of food causes them to move to where food is plentiful
2. A lack of water - they follow the rain
3. Climate makes living in one place good at some times of the year, but not others. Take the migration of birds. Birds travel tens of thousands of miles each year to go to favourable breeding grounds where predators are low, food is plentiful and there are suitable places to nest and breed.
In the case of wildebeest, they follow the monsoon. There is neither enough water nor food to sustain them. Predators migrate with the wildebeest picking them off as they travel.