How does endometrial shedding happen in menarche which is an anovulatory cycle?

Estrogen: An ovulatory cycles are regulated by Estrogens alone ( Progesterone becomes a part of ovulatory cycles). So endometrial shedding happens under the influence of estrogens.

Runs out of gas: The endometrium is a temporary structure built up by estrogen stimulus but maintained during pregnancy by other hormones originating after conception.If ovulation occurs without conception, the hormone signals change and the endometrium sheds. Without ovulation, estrogen continues the build up, but the endometrium gets too old and breaks down eventually, often with irregular cycles.