Beets and chard (both Beta vulgaris) are biennials. This means they should put all their energy into vegetative growth in the first year, overwinter, then produce seed the second year. Every once in a while I plant some beets or chard that flowers in the first year instead of year two. I wasn't sure who to blame- me, the plant, the seed grower, or the plant breeder. It simply never crossed my mind to blame the weather.
All biennials require a period of vernalization in order to signal to the plant to go to seed. The trouble with some biennials is that the amount of time they require below ~50 degrees Fahrenheit might be a very short amount of time (a few weeks). Further, it is cumulative. So if you plant a biennial in the spring and it experiences enough cold during that time, it will "bolt" prematurely.
This means the length of time you can harvest leaves off of your chard is shortened significantly. So what can you do? Since the weather is becoming a bit more unpredictable these days, your best bet is to plant in succession from spring to summer to guarantee a harvest-able crop for longer, or hold off on planting till the weather is reliably warmer.