Put this way. Karma is a habit is true! Actually, since we have lived since countless lives, we have generated countless karma, white, black and neutral. Karma ripens base on condition. According to the Lamrim the strongest ripens first, the most familiar follows and the oldest last, depending on the condition generated. Since we have all the possible karma due to our countless lives and actions, it is basically the condition that ripens the karma.
So, what are the conditions? If one generate boddhicitta then the best karma that is similar to that motivation ripens. Why? Because, the condition for boddhicitta is the strongest one can generate. Doing Dharma work in the mandala of the Guru and any vows, prayers and work given by the Guru is with boddhicitta as motivation because we can rest assured that the Guru has boddhicitta as motivation. Even if one does the work with self interest, to look good, it still has boddhicitta as motivation, based on the power of the Guru's motivation because the result is to benefit others. On the hand, broken samaya and schism can be said as the most negative of condition that can cause the most negative of karma to ripen. In the 50 verse of Guru Devotion, it is said that to think negatively of the Guru can lead one to the deepest of hells ie Avichi. It is not the Guru that "sentenced" one to the hells but the our own karma. And we created the condition for that karma to ripen.
If one generally keeps clean samaya with the Guru, then the karma that is the most familiar ripens. If one is lazy, then our "laziness karma" ripens. Karma will ripen to block out practice to keep being lazy. Obstacles will arise that will keep us from needing to work hard and that will cause one to be in danger of breaking Guru samaya by not completing our work. If one is hard working then the karma that is the most familiar with that will ripen and we will work hard and keep our samaya clean.
It these way, karma is a habit.