Why an lh/fsh spike may occur using the estrogen feedback loop

An LH spike occurs during oestrogen feedback loop because the pituitary gland  positively responds to the high level of oestrogen and produces more LH.

During the ovulatory phase the amount of hormones like oestrogen, follicle stimulating hormone and Leutenising hormone are high. FSH enables the formation of follicles and LH enables the rupture of matured follicle and release of ovum into the oviduct.

In the middle of menstrual cycle when the amount of oestrogen is high pituitary gland give a positive feedback by producing more LH.  

This is called LH surge. By this time primary follicles would have matured into graffian follicle and would be holding the ovum. LH surge causes the follicle to rupture and the ovum gets released into the oviduct.