Yes, it was 10 aircrafts bought in one batch, so all they have C-checks in the same time. I think it's not so fair to ground it in this quantity, but Aviation Comission thinks differently :(
Image drops from 70 to 6 points, incomes drop too... it's very difficult to correct this situation then...
you should watch when your C checks are coming up by clicking on the aircraft tab and scroll halfway down the page. It list all C checks coming up in the next 30 days. If you see you have 5 planes due for C checks, start them up early so they are not going in 5 days apart.
Also, I buy the airplanes in batches of 2, then wait a bit and buy 2 more ... sometimes there are months in between but its worth it because you never have to worry that all your planes will get grounded at the same time ...