No contractor ought ever be reliant on a single customer. With well over 100 MSCs, spread out the work. Just as an MSC should not be dependent upon one client, the shopper should not be dependent on one MSC. Do not schedule 20 shops with one unless one also has 20 shops (or 40 shops) scheduled with other MSCs.
That said, there are many reasons for such bulk cancellations:
Perhaps the client is going out of business.
Perhaps the client wishes to cut costs in order to keep from going out of business.
Perhaps the client has decided that MS is not beneficial or simply not economically justifiable.
Perhaps the client has not been paying the bills and the MSC decided to stop extending it credit.
Perhaps the client has been purchased by another firm or is simply under new management.
Perhaps the client wishes different data and is redesigning the instructions/questioner.
Perhaps the client and MSC had a fight and the client has decided to go with a different MSC.
Perhaps the MSC is about to go out of business and the client got wind of it.