Is it possible that they just reached the right size based on your flow, light, etc around the same time your phosphates increased? 3-4" is pretty big.
Well anything is possible, but it happened with about a dozen different corals. I wish I had more pics of other corals, but here is an example, these pics are 2.5 months apart, the first was around the time I started phosphate dosing. This is more growth than I had in the previous year at least, probably longer.