No, it is usually due to really large png files and/or custom monitors that some webhosts have in place to keep an eye on long-running processes. Otherwise, the bulk process rarely ever stalls anymore. I’ve had it run for hours without interruption on servers where the max_execution_time was still set to the default 30 seconds.
The reason it is possible to run so long, is that php does not include any time spent running external commands via exec(), plus the fact that each image is run in a separate process via ajax, so it is actually controlled by your browser, not the server. The only other possibility is if you are losing connectivity to your server briefly, and your browser misses something with the ajax, but errors of that nature would be very random and sporadic.