As per my understanding, you are trying to find the mean of grid at each layer. In your case, if the output of rain(y,x,t) is a scalar, then calculating mean of a scalar is like reassigning. It is similar to
meanprecip(y,x,t) = rain(y,x,t);
If you want to calculate the mean of the grid at each year,
meanprecip(t) = mean(rain(:,:,t))
Moreover, your code is not an endless loop.