BWIA traded the 727s for 4 707s from Braniff as they wanted to increase capacity and begin long-haul routes to YYZ and LGW, later LHR. Of course, doing so meant that a number of regional services were cut as the 707 planes could not land in some islands, eg Cayman Islands and Tobago. Still a rather unusual decision by any means (why do you think BI gave them 4 707s?).
Hop to it, fly for life!