The dominant ignorance leads to the karmic cycle. Because of this ignorance, you have karma. Because of karma, your ignorance gets stronger. It covers the rigpa more. The sequence begins with ignorance in Tantra as well as in Sutra. For example, the twelve nidanas begin with ignorance. Because of ignorance, we commit wrong actions such as harming oneself or others. Therefore, ignorance comes first in the sequence.

Because we have gone through the cycle of the twelve nidanas so many times, often we feel that it comes from karma. Because of this, we have further ignorance and go around the loop again. So the cycle begins with ignorance, which can be intensified further with negative karma.

-- 7th Dzogchen Ponlop Rinpoche