When did you put your red onion sets in ?
Onions like most plants respond to the lengthening daylight hours& heat . flowering.
Couple this with a fair amount of water /rain and a very fertile recently manured ground then the conditions for bolting are ideal .
I over wintered my reds on ground that was fed with composted manure 18 months ago , so far out of 50 sets only seven have broken to flower but a lot of the onions are skinny with too much tops on them .
The conclusion I come to is .... Too much water & too much food ,as they are in full sun all day . I can control, the water and food angle ..this autumn I'll try again without adding the compost again nor will I be giving them a feed of fish blood & bone as a top ... side dressing.
I'll also try a late spring planting for some of the red sets rather than putting them in theb onion beds at the same time as the Sturton giants so that they have half a chance of getting less rain during the summer as well