An online etymology dictionary has the following definition -
defection 1540s, "action of failing;" 1550s, "action of deserting a party, leader, etc." from L. defectionem (nom. defectio) "desertion, revolt, failure," noun of action from pp. stem of deficere (see DEFICIENT (Cf. deficient)). Originally used often of faith.
Religion was the big defining thing in the 16th & 17th centuries. People might 'defect' for primarily that reason - as the Pilgrims Fathers did when they quit England for N. America in the year 1607.
Since the Enlightenment politics has replaced religion as the most likely reason for 'defection', by the claiming of political asylum. (The protection, by a sovereign state, of a person who is persecuted in his own country for his political opinions or activity.)
Finding chessplayers who fit the bill for being the "first chess defector" is not going to be simple and straightforward.