The force on the pendulum is opposite to its position. (1/2 cycle out)
The velocity changes in response to the force ; increasing when the force is positive, decreasing when it's negative - and so lags 1/4 cycle behind it.
And position is similarly the result of changes in velocity - another 1/4 cycle lag.
1/2+1/4+1/4=? So what?
This type of movement is called Simple Harmonic Motion