I had the same exact problem.
There are some pretty good tips in marv's journal on all this. Not as good as a specific evaluation by a real pro of course but there are good ways and bad ways to work on mobility.
The way the back rounds out at the bottom and these other examples is why JUST doing the exercise will not ALWAYS work if you don't know the difference between bad mobility and good. Some people say that all you need to do to squat deep is squat. In a perfect world where all your muscles were at the same exact range of tightness that would be true! But of course that is never the case. Now, squatting type drills are a part of it but just squatting under load won't result in a perfect controlled mobility.
Basically, if you have strong muscle and weak muscles in an imbalance then the strong muscles will continue to get stronger and overworked while the weak muscles continue to get weaker UNLESS you do something specific to correct that. Flexibility is a part of the same coin. If you have tight muscles and loose muscles the loose ones will cointinue to give you the range of motion the tight muscles can't, probably to your detriment. OR I should say tight joints because it is really about the ROM of the whole joint but specific muscles can be a part of that.
So if something lacks mobility something else has to compensate. It could be a joint under or above or both. But remember that your lower back isn't meant to have a huge degree of mobility. It needs stabibility. It's your hips that should be mobile.
Hits, I just shot Anuj this page with a vid of a good extension.
Watch the vid under HIP EXTENSION. That is the way it should look except a nice pause at the top would be good.
IGNORE the "back extension" vid and don't worry about the distinction between the excercise names. Call it what you want but do it with hip extension. Then if you do the statics that will work further on back endurance as well as overall core endurance. This is what your back does and is meant to do under load. Again, I try to never use the term 'hyperextensions' and of course the back stays in the same position during the whole thing and does not hyperextend at the top.