I spent several hours -- not to say days -- on researching of this problem (tried different Macs (about 15-20) at different stores, simulated various load and working conditions). Here is what I have got:
It happens on every Macbook Pro 13'' 2015.
Talking about the reasons:
One may notice that it happens only when your Mac is not connected to the power source. Sudden fast spinning never happens when your Mac is on charge or connected to the adapter.
I analyzed temperature sensors data (via TG pro or iStats) and came to conclusion that it is connected to the frequency with which SMC reads data from temperature sensors. This frequency is much slower when Mac is working on battery, so SMC is not able to detect temperature raise immediately, and, when working on battery, fans turn on only when CPU reaches temperature of about 95 degrees celsius.
Maybe, Apple engineers intentionally lowered sensor update frequency while working on battery in order to save battery life and to get this enormous numbers of 10+ hours on battery.
In most working conditions this behaviour is just fine, because on Mac model fans are completely off during normal operation, and came on only when it is needed. Yes, it is somewhat frustrating if your fans are suddenly come on when your Mac is working on battery and under heavy load , but it is completely harmless for your computer.