I work in the intermodal side of the business, and let me just say this: It mostly has to do with economics.
Hunt, Schneider, Werner, etc. most likely have contracts with all of the major class I's. Depending on the contracts and the linehaul pricing, IMC's (Intermodal Marketing Companies) or the logistics arm of the asset based trucking companies (Bi-Modal carriers) will chose the routing primarily based on cost.
If you're seeing an increase of traffic from GRS to CSX, most likely CSX dropped their rates or are offering a short-term incentive to increase traffic into Worcester.
Keep in mind the intermodal networks as well, you can't ship into or out of Ayer and Worcester from every origin terminal on the CSX, NS, BNSF, or UP network. If you have a steady increase of inbound freight to New England from a point served by the CSX intermodal network, but not the NS, the IMC's and Bi-Modal companies will obviously chose CSX.
Personally, I prefer service on the B&A / CSX over the GRS / NS in Ayer, but that is just me. However, often it is the origin terminal, and economics which is the true determining factor on which end terminal I end up using, rather than service and preference.