I wonder if this was always the plan?
About 3 years ago, having been with 'free' PB for a long time, I used up the 'free' bandwidth and paid for another 10GB. Entirely fine.
At the same time I enquired about using photobucket as a proper hosting site for my image galleries, but the only paid account option they had still showed adverts to visitors - there was no 'pro' gallery option to showcase customer's work. On that basis I continued using them to host images on other sites, but there was no future for them as a pro gallery, so I looked elsewhere.
When I have had the misfortune to visit using a browser without ad blocking, the experience has not been pleasant. When visiting as I have been, with decreasing frequency recently, there have been nagging popups asking that I consider paying extra for the bandwidth I'm using, and I very nearly succumbed, except that I barely use them now.
I'm grateful for the years of free image hosting, but can only consider that they have just decided that they don't want to continue running the site any more.