If I had to wager a guess, I'd say in her first life she fell for the 'Demon King' and he likewise fell for her, and the 'Hero' of that life wanted her for himself. When she didn't reciprocate the feelings the 'Goddess' sided with the 'Hero' and placed a reincarnation curse on her that would only break if she was to find the 'Demon King' and they were to fall in love again. Thus the 'Hero' of each life is tasked with killing the 'Demon King' before they can meet and she gets back whatever powers she had.
As for her powers, I'd say that she was originally 'Isekai'd' here for one of the standard reasons, and gained fabulous power because of it. However, when she fell for the 'Demon King', rather than doing as the Goddess wanted and destroying him, she was angered and sealed her powers away. Only evidence I have for this part of the theory is page 16 of chapter 11 were she can be seen with a girl in a school uniform.