Many herbaceous shrubs,flowerd,fruits & vegetable have come on early or bolted this year,it would seem.(They have in our garden).
I think that this not due to overfeeding,but rather should be put down to the weather conditions.
We have (in East Sussex anyway) had alternating periods of (very) hot weather and cool but very wet weather.
This has (in my opinion) cause the plants to grow unaturally fast,and sometimes (as with your onions) to bolt.
I don't think there is much you can do to stop it,we just have to get used to these odd weather conditions,and try to allow for it.