>>8470893It's due to the way the hilt is designed, to activate the saber with your thumb you would have to hold it from the bottom. Anakin could adjust his grip after this but doesn't (although I think he does this in the prequels). In my headcanon he does it on purpose to throws his opponents off, switch from right sided dominant attacks to lefts sided dominant attacks, sort of like his sabers ability to adjust the blade length on the fly, a simple trick to get an edge.