In AC RL circuit why does current increase when voltage decreases?
The diagram is for the pure inductance only. In other words, the voltage shown there is the voltage across the inductor only, and the current is only the inductor current. These values are always 90&deg; out of phase.

When considering the series or parallel combination of an inductor with a resistor, it gets a little more complicated. In a series circuit, the current through both components is identical, but the voltages have different magnitudes and phases, and their sum has a phase angle of somewhere between 0&deg; and 90&deg; relative to the current.

Similarly, in a parallel circuit, the voltage across both components is identical, but the currents have different magnitudes and phases, and their sum has a phase angle of somewhere between 0&deg; and 90&deg; relative to the voltage.