Q:

How can I warn another team that the boss is going to call them out? 

Here's what has happened so far:

Team X is supposed to do something for us, and the timing was kind of vague
but it's been a long time now. 
The boss said he was going to send an email to them cc'ing their boss and their boss and demand a commitment date
As team X will need some information from us to do the thing, I
emailed them with some details they'll need to do it, without any
mention of dates or anything. 
Team X cordially gets back with a TLDR, and they'll get to it eventually. 

Now I'm afraid my boss is going to send that email and they'll think I set them up for an ambush or something. 
How do I warn them they're probably going to get hit by a torpedo without them thinking I'm in on it? 

A:

Actually, nothing is preventing you from sending an email saying:

Hey Bob, this project is becoming more urgent for our team. 
Could you guys please commit to a date for us? 

And maybe you could give the other team a quick phone call as well in addition to the email. 
Then, if they send a date back, you quickly forward it to your boss, and you tell him as well in case he doesn't see the message in time. 
Of course, it's possible that you'll be too late, or that the other team responds too slowly.  Sometimes, there are just too many variables out of your control. 
PS: Thanks to ObscureOwl for finding the better turn of phrase. 

A:

Don't tell them.  Tell your boss. 
It's not your place to warn them about your boss, and it could tend to undercut him, so don't do it. 
Instead, explain to your boss your concerns.  Tell them what you just told us-- that you only recently sent them an email, and that you're concerned that your boss's actions will be tied back to you and that they will feel ambushed.  He may take a slightly different approach, or at the very least should be able to word the email in such a way that you will not be accountable for your boss' actions.   

A:

How do I warn them they're probably going to get hit by a torpedo
  without them thinking I'm in on it? 

By not getting involved in the first place.  It doesn't seem like it is in your place to manage the other team, or that you are responsible for their (lack of) delivery, and as such, you should've stayed away from it in the first place.  If you know someone personally on team X then you could mention the tension in private, but for sure not send them an unsolicited mass email. 
As it is I would just leave it alone.  I don't think there is anything you can say to change the mind of those who will think you caused the call-out, and right now, at least, they cannot be sure that you knew about the upcoming bomb.  Once you start apologizing/retracting/whatever, there will be no denying that you knew about it and some people may resent it for not warning them, for example (I know, it's ironic).  While you cannot undo your previous actions, now it's time to just stay quiet and not get involved more than that. 