I was wondering about that timing, too. Possibly because something in their car counts or accounting caught somebody's attention?
Gotta be money. Call me cynical, class 1 railroads don't do anything that doesn't make them (lots of) money.
Penn Central worked interchange to their advantage, using their expansive reach to detour tonnage away from EL, L&HR and others for straight PC routings. UP is likely worried about CP's plans in this regard. Even if CP keeps UP interchange open, there are ways to purposely impair transit times and give Shippers cause to turn away from UP. Perhaps it was a Shipper that alerted UP to action?