The amount of ambient light can vary greatly at night, depending on the availability of moonlight and starlight, which in turn can depend on the amount of cloud cover or tree cover. In low light your depth perception falls off, so walking on highly uneven ground can result in the sort of jolting you get when you step off a curb without noticing. Obstacles, such as rocks or roots are harder to see and can trip you.

The easiest solution is to walk on relatively familiar and well-maintained paths. The next easiest is to carry an LED light you can dim way down and carry it low, away from your eyes, turning it on only when you need some extra assistance. It is said that red light won't ruin your night vision as much as white light, but I haven't really verified this for myself.