I've had three F-150s and have had a plug blow out of the current one, I've also personally seen it happen to a couple of other Ford trucks. The issue is from a couple of different things, 1, there are only 4 threads in the head holding the plug, 2, the plugs aren't torqued tight enough, 3, they are supposed to be changed at 100,000 miles and nobody checks them prior to that, so when they become loose they end up blowing out.
Plus Ford dealers want over $500.00 to just change the pugs, so people don't change them earlier than needed. it's a hassle to do them, the rear plugs can be very frustrating.
Ford say's there is no issue with the plugs, but it's a common problem with these heads. The recommended Ford repair includes removing the head and is very pricy.
The problem is so common there was even a company that specialized in the repair and would fly to your location and do the job.
Heli coils are not a good solution, a better product is a Time Sert, it's a solid insert and fairly easy to do yourself. I bought the kit for about $200.00 and it maybe took 45 minutes to do. All the needed tools and parts are supplied to do all 8 plugs. You ream out the old threads, rethread the head, epoxy (it threads in) in the new insert, then put in a new stock style plug. It took longer to get everything ready than to actually do the job.