Simplified answer that probably won't make you feel better, but hopefully at least makes it make more sense: the supply rate is the price of the electricity that National Grid buys and delivers to you. NG isn't allowed to profit from this charge, so it essentially reflects how expensive it is to generate electricity. About half of the power generation in New England is from natural gas, and because we're natural gas pipeline constrained, we're more dependent on liquefied natural gas (LNG) imports than basically anywhere else in the country. This makes our electricity rates really sensitive to the global LNG market - very bad when there's a war in Ukraine and the rest of Europe is looking for natural gas from non-Russian sources and there's suddenly a lot more demand for a more limited LNG supply. Then in the winter, you add the fact that we're using natural gas for heating too, so both domestic and international demand are higher than in summer or fall, further driving up prices.

TL;DR high LNG demand -> high LNG prices -> high fuel cost for natural gas power generators -> high price to make electricity -> high electricity supply rates. There's more complicated nuances about like rate changes every 6 months and bids and contracts and energy futures but that's the gist of it. It's thankfully going down now in the spring/summer because natural gas prices are down, partly because there's less demand for it for heating.

As for whether you can switch to a different electricity supplier - you can, but I'm not sure which of the non-city suppliers, if any, are good/nonpredatory (I basically only ever see people warning against them).