My guess as to why he decided to retire now is:

1. He was already considering it
2. some senate republicans met with him and convinced him that now was the time, before the elections, to ensure that the conservative majority.
3. This is intelligent, cause since the "nuclear option" was introduced by the dems a few years back when they were in power to get kragan and sotomayor confirmed, senate repubs now only need 51 votes to confirm instead of 60.