That is a pretty complicated question. Without actually seeing your form I would just be guessing. Usually it is glute and hamstring weakness compared to the low back or just using too much weight. Glute and Ham exercises are good particularly the glute/ham raise. Also when doing deadlifts just focus on proper form and having a smooth pull without the hitch.

Using too much weight all the time can lead to bad lifting habits like Dieselweasel here.