Let's Define Tunneling
Basically, I find a lot of people tunneling, and while I think it is a dumb way to play, I do think that if you're going to, you need to know which one you're doing, including accidental, so here are my definitions.
Order Tunneling: The one I hear everyone claim to be what "tunneling" actually is. This one is when you hook the same person twice or three times in a row. This can technically be accidental since sometimes you just can't find anyone else and this person keeps showing up. I believe this is what Killers think when Survivors claim it.
Timed Tunneling: The one most often defended as "not tunneling" is Timed. This has nothing to do with hook order and has everything to do with how long until a player is downed and hooked again. This one can be accidental if Survivors look identical, but I still find it to be the MOST annoying. It is tunneling if you don't give someone time to breathe in between hooks. To not be doing this kind, a minimum of 30 seconds helps make it feel less like you're tunneling. I believe this is what Survivors are referring to when they usually claim it.
Forced Tunneling: This one Survivors cannot get mad at. Don't go looking for the Killer because you will end up getting tunneled if you keep poking them. I do not defend you if you try to get the Killer's attention. It's on you, and most Killers won't do it for the soul purpose of, "I almost have the strong one down, why should I whack the weak link?"
Accidental Tunneling: The last one is when you are doing any of the above, but you don't realise it. Typically it comes up when you have Survivors that look the same and when you can't find anyone else because they're all stealthing around the map instead of protecting the people they unhook.
There. Those are the definitions I could come up with. If you have any more, or wish to try and defend any on this short list (excluding Timed, because I know you don't think so if you're a Killer) feel free to.