Your onions are “bolting”, in other words, trying to flower and go to seed. The pointed tips are the flower buds. Flowering can be triggered by cold spells, drought, basically everything that stresses the plants, triggering the “go to seed to survive“ impulse. Or it can happen in sets that are so well supplied that they flower a year earlier than the usually biennial cycle.
If you are planting the onions for storage, you don’t want them to flower - it means they won’t grow further (at least not much) and they also don’t store too well. You can snap or cut off the flower stalks to stop the onions from putting even more energy into the flower, but it’s just somewhat mitigating the effect. If you are planning to use some “fresh”, e.g. in the spring onion state or similar, pick the bolting ones, leaving the non-flowering ones for storage.
If you are picking sets next year, don’t chose the big and fat ones, as they are more likely to bolt. About the size of a hazelnut, plump and firm is best. Sometimes you can get heat-treated sets, that were stored at temperatures between 25 and 30 C for up to three months, greatly reducing the chances of bolting. Picking the right cultivar also helps. And sufficient watering combined with a bit of protection in case of late cold spells also is a good idea if you planted early.
