Why were early aviators' trousers flared at the thigh?
It's not too hard to figure out.  Wikipedia says Jodhpurs originated from India where they were part fashion and part practicality (the loose fit in the upper half for horse riding comfort) and evolved into the uniform of most western cavalry officers by the turn of the century.

And, in WWI, where did most, or at least a lot, of the pilots come from in the early days?  Cavalry.