but the upper middle class is relatively small, depending on how you define upper middle class. Nationally, one might say the 15% who are below the 95th percentile (but locally is what matters for the purpose of buying property, and maybe "upper middle" is 60% to 95% of the local ranking).
For upper middle class people the answer is probably having a baby, so you can see why if people are having babies later in life when their incomes are higher, prices rise.