Depends on the machine. My machine requires an incident and the one and only time my Bernina went out of timing is because I forgot to change the foot and the needle came down on the incorrect foot I had installed. When my Innova went out of time my drive belt was going and after my husband replaced it he didn't reinstall something right so machine went out of time again and again. Once we corrected the underlying problem and retimed it I have not had an issue since but until we figured out what happened with the drive belt I had reset the timing at least 5 times. Thank goodness for Innova tech support because they finally figured it out over the phone at no cost to me.
But there was an underlying cause that the timing went out which was the point I was trying to make. It doesn't just happen for no reason, if that was the case every single sewing forum would be inundated with "help my timing went out" posts, for both domestic and longarm machines. If a machine goes out of timing for no reason I would consider that machine subpar. Also I would consider hitting a pin, a set screw backing off or a huge thread wrap an "incident". Hitting a pin or a ruler or a thread wrap the user would be aware of. A set screw backing off a user would not (except it should make a new noise in the machine and on my machine, at least, there would be a noticeable change in stitch quality before the machine failed to make a stitch, such as skipped stitches, loops in the stitches or bad/crooked looking stitches). Again an underlying issue. A sewing machine does not just go out of time without some cause. If the underlying cause is not identified and/or rectified retiming the machine will only be a temporary fix at best and make no difference in a worst case scenario.

One time, the machine bent the needle. I discovered, it wasn't inserted properly. Very easy fix-new needle installed correctly. It didn't mess up the timing.

I don't even know where to start ! First would be that changing your drive belt has absolutely nothing to do with timing. Your husband was at the wrong end of the machine to not "reinstall something right", and have it go out of time! It's beliefs like this that get said and believed, and it's unfortunate because it doesn't help people that need help.
Second.."My machine requires an incident" to get out of time...I have no words !
I would consider hitting a pin, a set screw backing off or a huge thread wrap an "incident" ....didn't I say that???
I've been timing and repairing machines for over 40 years..I stand behind what I said. Done.

There is a drive belt in older models of the innova that delivers power to the bobbin race from the drive shaft from the motor which is located at the back of the machine but the belt we replaced was at the front in what is called the "Hook Drive Mechanism". See page 19 in the attached link. http://www.abminternational.com/imag..._MANUAL_V5.pdf

The race doesn't rotate on it's own it needs to be powered from the motor. There is a drive belt under the machine behind the bobbin race mechanism that attaches to multiple pullies. I just checked my machine schematic diagram and they call it a "timing belt". I apologize if I used the incorrect term and confused you.