It would seem that this is due to a small group of users who have pretty much abused the service. Apparently these users have gone beyond what Microsoft has deemed as “reasonable use” when it comes to the unlimited storage, kind of like how some carriers such as T-Mobile aren’t thrilled when users abuse their unlimited data plans by downloading huge files.
This change in plans applies to Office 365 Home, Personal, and University, but it seems that for now Business users remain unaffected. This change in plan is expected to come into effect from early 2016 onwards where Microsoft will offer 1TB of storage instead. In addition to this, Microsoft will also be cutting back on storage limits on their other plans.
According to a statement made by the OneDrive Team, “Since we started to roll out unlimited cloud storage to Office 365 consumer subscribers, a small number of users backed up numerous PCs and stored entire movie collections and DVR recordings. In some instances, this exceeded 75 TB per user or 14,000 times the average.”