Interesting, what happens if connections are frequent and reliable?
Does this drop still happen, or is it a reflection of peoples unwillingness to be stranded at an arbitrary station of no value to them?
From me perspective, one or two connections that are frequent|reliable is not an issue; especially if toilets/refreshments are available.
I have no data correlating traffic drop against connection reliability - sorry - best I could do is educated guess.
Biggest drop of all at Drayton Park could relate to it being very close to West Ealing - many homes will be within walking distance of both, and people who walked to Drayton Park before for a direct Paddington train are probably walking to West Ealing now.