Probably because it is using up a good chunk of resources. A staff member can check the logs to see what is triggering the suspension (down to the file generally), but what you should first do is to check to make sure any caching or AJAX functions of the software are turned off. They can use a lot of resources at times. It's also a good idea to check any plug-ins you might have installed too.
I might also mention too, if you're getting a ton of hits in a short amount of time it might be pushing you over the resource limit for Free as well. Software can generally be tweaked to prevent that to a point, so it's depending on the circumstances and how Moodle works (I've never used it).