I can only surmise myself as why that may be the case, if indeed it is.
First there is typically one start point on the east, Yorktown, whereas on the west you have Florence, Newport, and Astoria. It seems the majority starting in the east start around the first couple weeks in May and their first three or four overnights center around Willis Church, Ashland RV campground and then Mineral. So there are these opportunities to connect early on.
For the first 10-11 days going west it tests your endurance, especially Afton and on, causing short days and rests days for many. This generates opportunities to play leap frog and frequent meet ups. Plus everyone seems to shoot for the known hospitality stopovers like Troutdale, The Place, Rosedale, Outlook, and Hindman for example.
I have followed maybe 8-10 CGOAB TA journals for 2015. However there are not that many west to east journals in the mix this year so I have limited data there. I'm only looking at 2015 to sort out an itinerary for myself for a possible 2016 run at the TA again.