Equation for square law circuit modulator
2fm
This frequency comes from the message signal; specifically \$(m_{(t)})^2\$. Multiplication of a signal with another (or itself) in time domain is represented in frequency domain as a convolution.
So the corresponding signal in frequency domain is
$$
M(s) \circledast M(s)
$$
If a signal with frequency contents from 0 to \$f_{max}\$ is convolved with itself, the resulting spectrum will have frequency content from 0 to \$2f_{max}\$.
From Wikipedia