Reason menorrhagia caused by fibroids?

Few causes: If the fibroids are growing inside the lining of the endometrial cavity (called submucosal) they cause irritation of the lining within the uterus, and they have their own, anomalous, blood vessels growing on the fibroid that can cause bleeding. If the fibroids grow in the muscle layer (and are large, or very numerous) they can disrupt the normal blood supply to the lining and cause heavy periods.

Fibroids: Fibroids need blood to grow and so they collect vessels to their surface which can be more likely to break and bleed at the time of your period. Also fibroids prevent the uterus from contracting down and stopping bleeding vessels during your period, leading to greater overall blood loss.