So you had a corruption of a file or your hosting account..... Static, inanimate objects (such code) do not just suddnely decide to change or break at random points in time, just to annoy users. Somehting must have happened to change the behaviour. But of course, the first port of call is the Joomla! forums, not the provider or others maybe having access to the file server. These things happen, correct the issue and move on, it's hosting, it's unix and it's out of your control.
In many cases, this sort of thing happens if the server has some form of hiccup, panic's, reboots, or in the worst instances, disk failures or corruptions and rogue or damaged write processes. So again, maybe the title of the post needs to be changed. If, as you state in your defence, that you upgraded to J! 1.5.7 after the problem occured, how is this "linked" to J! 1.5.7 update.? <shaking head>
What server process could be responsible for this?
Any process running on "your hosts" server that had write/change access to your hosting account.
This could have been a one-time-event, btu if it happens again, talk to your host about the stability of their services or if they added a new service/process/application around that time, if they were working on the server at that time, if the local leopard in the Zoo changed it's spots at that time, if martians landed in or around the hosts data centre just before the occurance...!!
<unsubscribing> this is going nowwhere and is a futile excercise in will-power.