> Any particular reason/mechanism why the Higgs field suddenly (gradually?) switched on?
"Switched on" is not really a good description. According to my understanding of our best current model, the Higgs field was not in its vacuum state in the very early universe--there were lots of Higgs particles around--so it was not "switched off" any more than any of the other Standard Model fields were. But in the very early universe, the electroweak interaction worked differently than it does now. As the universe cooled, there was a phase transition that changed how the electroweak interaction worked, and after that phase transition, the Higgs field acquired what is called a nonzero "vacuum expectation value", meaning that even though there were no longer any Higgs particles around-- the Higgs field was in its vacuum state--that vacuum state now corresponded to a nonzero value of the Higgs field, meaning that the field can interact with other fields, and that interaction is what we observe as mass for those other fields.
"Switched on" is not really a good description. According to my understanding of our best current model, the Higgs field was not in its vacuum state in the very early universe--there were lots of Higgs particles around--so it was not "switched off" any more than any of the other Standard Model fields were. But in the very early universe, the electroweak interaction worked differently than it does now. As the universe cooled, there was a phase transition that changed how the electroweak interaction worked, and after that phase transition, the Higgs field acquired what is called a nonzero "vacuum expectation value", meaning that even though there were no longer any Higgs particles around-- the Higgs field was in its vacuum state--that vacuum state now corresponded to a nonzero value of the Higgs field, meaning that the field can interact with other fields, and that interaction is what we observe as mass for those other fields.