The explanation is extrenmely simple: It depends from where you are looking into the resistor network. Looking into the output node we see three parallel pathes to ground. Hence, the resistors of these pathes are to be considered in parallel.
Another example: In your circuit, take the two resistors R1 and R2 - without all the other parts .
Are they connected in series? Answer : Not possible without additional information.
It depends from where you are looking into this small "network".
(1) Looking from the left or from the right (the other end grounded) - they are, indeed, in series.
(2) Lookinng into the midpoint (both ends grounded), they are in parallel.