One of the following have probably happened:
a) Posted in irrelevant groups, reported by members of the group
b) Posted too many times in a short time period, causing it to be automatically flagged as spam
c) Due to the title the link may be seen as “fake advertising” by mistake due to Facebooks algorithm
Try to avoid posting the same links to many times. Also avoid posting more than 4-5 times per day (in different groups, not the same one), and spread your posts throughout the day.
This should help you avoid being reported or flagged by the algorithm.
Also, keep in mind that if your account has been flagged with spam recently, Facebook will keep a closer eye on you (automatically) for the next 30 days, and you will quickly be flagged again (and banned for a time period) if you keep on posting.