I think Lear has a mental breakdown because of the pressure. It is not uncommon for a person who is under a great deal of stress and has a lot of responsibility. I think I might go off the deep end too if I was head of Lear’s kingdom.
There is a definite sense in which these two separate phenomena are paralleled in this play. In a sense, the play is the story of one man and one kingdom's descent into anarchy and lawlesness. Lear's decision to remove himself from his position and the way he is subsequently treated by his ungrateful daughters causes him to experience a personal form of insanity, just as the jockeying for power and treachery of his daughters and Edmund causes the kingdom to experience a period of massive disorder and anarchy. The two are definitely paralleled.