I've had that happen occasionally - it's because something causes the top thread to catch and stop advancing. The top thread keeps plunging up and down catching up the bottom thread. There is too much tension on the thread to allow the fabric to advance properly. Eventually the tension on that top thread is enough to cause the thread to break - or the needle. Once the bent needle plunged down into my bobbin casing and hit something it shouldn't hit and damaged the casing. So, I've become very aware of when that starts, and stop sewing promptly.
I've found two reasons why it happened: First, sometimes a nub or a broken ply would bunch up and cause the thread to catch in the needle eye. Second, something, possibly a bit of debris, in my tensioning plates would catch (I never actually saw such debris, but once or twice when my machine jammed up like that I could feel that the thread wasn't running through the tensioner smoothly, so I rethreaded and it worked.) For the first problem, you could look for a source of more reliable thread, or just stay really aware of the issue and stop sewing as soon as it starts sounding funny.
For the second, which sounds more likely in your case, when you clean down the machine make sure you clean the tensioner also. I use a can of compressed air for deep cleaning my machines. Buy it in a computer store, not a sewing store (or the electronics department of WalMart instead of the craft department.) Same stuff, but it's much cheaper and doesn't have nasty "lemony fresh" scents that could possibly leave residue on your machine.
I brush down all the reachable areas every time I fire up one of my machines. I use the compressed air periodically, say every other day when I'm really on a sewing binge. When I'm only sewing occasionally, I try to clean it like that every time because otherwise I'll never remember when I cleaned it last and how many times I've used it since then and it'll just be neglected...
<OT musing>I keep meaning to get a whiteboard to put in my sewing room where I can write down maintenance things, and stuff like how long individual sewing sessions are so I'll know if my needle needs changing. Currently I change out my needle almost every time I sit down because I can't remember how long I'd used the previous one, and I'm sure most of them are thrown away with like one hour of use on them...</OT musing>
