To much build up pressure in coolant system!

Those are core plugs, not freeze plugs, frost plugs or some kind of block protection scheme. Very rarely, IF at all did they actually prevent block damage from freezing. When the block mold is poured, you need an access point to get the core casting sand out. Picks, rapping, vibrators all were used to quickly get the sand out, then the holes machined for a water tight seal. Sure someone at sometime got lucky, it would be neater still where the "history" of freeze plugs started; but it's not their intent.