Why does farm labour cost so much more at sowing and harvest peaks?

It is simple supply and demand. At sowing and harvest almost every farmer in the area needs hands in the same short window, so demand spikes while the supply of workers is fixed, and the rate jumps. The work is also harder and more urgent, and workers know your crop will suffer if it waits, so they can command more. You cannot change the market, but you can reduce your exposure: book your regular gang or a contractor in advance with an advance so your rate is locked before the spike; stagger your operations by using two or three maturity windows so you do not need everyone at once; mechanise the peak operations like harvesting and transplanting where a machine replaces a scarce gang; and be a loyal employer so your team comes to you first rather than chasing the highest bidder. The farmers who plan labour early ride out the peak; those who wait pay the most.