Probably because they were added to the missing thumbnails pool (pool #81). An automatic job task runs periodically to create new thumbnails for pictures in that pool, I believe.
Posted under General
Bump for more clarification.
post #806264 had a working thumbnail for me, then after I left a comment on it, its thumbnail was broken. I'd run into posts whose comments indicated that the thumbnail was broken for some and not for others before and always assumed it was due to browser caching, but that can't explain this. What's going on?
Derp. I had ruled that out since http://hijiribe.donmai.us/post/index?tags=shiroaisa and http://sonohara.donmai.us/post/index?tags=shiroaisa both have the same thumbnail, but now that I look, that's because they both take images from danbooru.donmai.us rather than using their own domain names exclusively.
So does this mean that the batch script that runs on pool #81 is set for different times on different servers, or has albert just been fixing them manually? I know that some images in that pool have been fixed on both servers in the past seven months.