There could be a few possible reasons...
1) you're simply too weak
2) you don't know how to activate leg drive in the starting position of the deadlift or you have weak legs
3) it is simply a result of your leverages
This are just some of the reasons I can think of right now.
As for solutions...
1) get stronger overall
2) start doing some deadlift variations or assistance work to focus on bringing up whatever muscle group that's supposedly lagging
Do post a video of your deadlift and provide us with some information such as how long you've been training for and what weight are you deadlifting, at what bodyweight etc.