Because you sit too much!!
How many hours per day to you spend sitting and how long have you been doing this for: years of work, TV, driving even socialising we are often sitting down
1)Your hips are tight
Sitting tightens the muscles at the front of your hips and weakens the muscles in your butt (because they are stretched out). You are also likely to get a very stiff back from sitting for long periods – that is why it can be hard to straighten up at the end of a long day sitting.
Try standing up regularly though out the day to stretch your hips and back – this will help prevent these imbalances from occurring.
2) Your hips are weak
Muscles that aren’t used get weak. Tight muscles are often also weak muscles because they are not getting used.
So yes, we do need to stretch our hip flexors, but we need to strengthen them too – this is an often-missed part of rehabilitation that is very important.
Often you think you are strong but maybe you are only strong in your big mover muscles like the quadriceps in the legs and your little stabiliser muscles that lie deep in around the hip joint are weak. We must not forget those small muscles when we are rehabilitating the hip