Same thing happened to me initially. My hamstrings would tense. This is most likely due to preferentially activation of hamstrings over your glutes. Your hamstrings are doing all the work and your glutes are inactive. Its a muscular imbalance issue. Try smashing and stretching the hip flexors first prior to the exercise (this will allow you more hip extension). Then when you perform the exerciser try pushing through your heels, and focusing on lifting with your glutes. Focus on contract the glutes while performing the exercise. It may take awhile before your motor patterns are corrected.