I accidentally pulled the cable out of the slide valve or the slide switch.
This occurs when when too much tension is applied to either the valve or control assembly. These are designed to allow the cable to detach, instead of breaking the mechanical assembly.
If you pulled hard and the cable couplings slipped, then the gate may have become misaligned, allowing concentrated oxygen to mix with low-altitude air, or vice versa.
This tutorial illustrates how to align the gate, and reconnect the cable.