Why is this relay turning on when booting up/down my raspberry PI?
If the relay module needs pulling high to turn it off, then you need a pull-up resistor to 3.3V, not a pull-down resistor to GND.  I'm not sure what the 1K resistor is for.
On most microprocessors, a GPIO pin can be in one of three states:

Output low
Output high
Input, high impedance

(Because it's a GPIO pin, not a GPO pin).
On powering up, the safest option is the third - high impedance.  That way, if the pin is intended to be inputting a signal from another system, the processor won't mess things up by pulling the GPIO pin high or low, and damaging either itself or the circuit that's driving it.