I have noticed this backup every day since they opened the new express lanes west of the 410. I am not sure, but it seems to me it could be any of the following:
1) Lane realignment. This always causes confusion and a backlog in the area in some way, but usually leading UP TO the realignment.
2) Busy past Eglington. The 401 EB seems to be busy from Eglington to the 400 everyday (don't know about weekends). Not sure why it has been busy in this section, literally EVERYDAY, but maybe it has to do with cottage goers taking the 400 north? People are of course forced onto the express lanes coming up to the 427 from the EB 401 and into another jam when they reach Eglington, not too far down.
I think once people get used to the fact that there is now an express/collectors system in the area and they do not get a chance to get into the express until the 427 area, more people will take the express and loosen the jam.
I dunno, that is my completely non-scientific explination that could be completely wrong.