Honestly, it would not surprise me in the least. It's either the NSA or the chinese. I remember reading a story that the military couldn't order electronics from chinese manufacturers anymore because of all the random backdoors they had built into the hardware.
Regardless, it's not too important who it is. The fact that it was done at all should be concerning.
Juniper Networks is one of the largest networking companies in the world, and undoubtedly has some of the most efficient Cyber-security systems on the market. In order for a backdoor, let alone two, to be placed in their firewalls, it would have had to be done with their knowledge. The NSA is the most likely organization to have that kind of access to Jupiter Networks firewalls. They are also large enough to have the strength to manipulate a company as large as Juniper into allowing it.
It's absolutely possible that the NSA is behind the unauthorized code found in the Juniper firewalls. However there's no concrete proof that's the case. It could also be another entity, or group and not the NSA. Either way, someone certainly can benefit from the breach, and it's not the first time the NSA has been accused of doing something like it.
If Snowden was able to inform us that the NSA has physical back-doors installed in computer hardware that they intercept during delivery, then there is nothing much that I could imagine that they wouldn't do. The NSA is a shadow-government-type body that is entirely unconstitutional. It violates the liberties of everyone in the world who is connected to the web.
Thanks to Edward Snowden blowing the whistle on the NSA the American people have become more cautious and aware of who or what may be monitoring them. It would make sense for them to be behind this little conspiracy as they have a bit of reputation of spying on people as of late and this fits their MO. Not saying this couldn't be anyone else but the NSA should not be ruled out, especially since all signs seem to be pointing in their direction.