Upon further reflection, I'll bet I have a little understanding. If you look at the schedule, the last train in the AM 'rush hour' block is P586. This is probably the standard they have been using for deciding when to start track work & when to start delaying trains as a result. Hence P512 gets delayed as the first official "mid-day" train.
I don't think that really matches current perceptions, usage, or ridership levels on the Framingham-Worcester line - P512 still seems to be a very popular train with people commuting to work. On the former (pre-May 2016) schedule, the 8:40 AM departure from Worcester (same as current P512) had 237 passengers. Not a lot, but still a couple of hundred passengers potentially being delayed for construction. Compare that with ridership on the next train (10:40 AM departure from Worcester) which only has 133 passengers. [Both those passenger count values are from June 2015, so they are probably not accurate. But it's all I've got. And some Worcester passengers from the pre-May 8:40 AM Worcester departure may have moved to the #HeartToHub train, but that's not an option for the Grafton to Newton passengers.]
In addition, the collection of ties is not as intensive a construction project as the tie replacement and rail surfacing tasks that have been recently underway. So perhaps it is easier to delay the tie collection work for 30-45 minutes. In other words, I doubt we would have had the same success having this suggestion implemented back in June for changing the start of the tie replacement work (I actually did make the P512-P507 priority comment previously).
But what a great response from Keolis!