Which programs interrupted?

It's curious to see that WUNC interrupts some programs during the fund drive and not others. I wonder if it is a function of the program supplier? It sure would be nice for WUNC to explain more, and more often, how they go about picking the programming they obtain and choose to carry on their frequencies. That represents about 90% of each 24 hour period, no? It probably even includes the news promos read by local announcers, at least the words, but I am only guessing at that.