Personally, I do not believe that OPNP nominees getting a call from Opportunities Ontario is because of the CIC's QA exercise, and I say that because similar calls were being randomly made last year as well to OPNP nominees, when there was no QA going on at all. I think it's because Opportunities Ontario wants to keep measuring the efficiency of their PNP criteria and ensure that the very best and genuine folks are selected for nomination in future. Especially, in the case of Masters/PhD graduates stream since it's a without job offer stream and still running as a pilot only.
Bear in mind that Opportunities Ontario has a right to contact you even AFTER you receive your PR, to make sure that you're still residing in Ontario and have not misused their program by getting a job in another province.