Asking about a colleagues post-grad work? This may not be the best place to ask this question, if so please inform me. 

I am approaching the end of my program and will be applying for jobs soon. But some of my colleagues (some of which I'd even go as far as to call friends) have already decided their industry, company and even signed offer letters. 

But when I ask details about the company, the salary, the benefits or scope of work. Some people get really defensive. 

I don't understand why this is. Is this an academica thing, a culture thing, or am I just a bit more socially inept than I thought? 

From my perspective as someone who is also soon graduating asking for insight on such things seems perfectly normal.
It's perfectly fine for people to be willing to share more "closely-held" details, just as it is perfectly fine for people to be unwilling to share. 

Let's take the position of the person you are asking:


they might not want to tell you the salary, because it feels freakishly large to them after living on a grad stipend (or even compared to common academic salaries), and they don't want to make you feel bad or envious
the salary is low compared to what they hoped or thought would be common, and they don't want you to think less of their success or options
the person isn't sure they are supposed to share the non-public hiring data, and most places don't tell you in the hiring process "hey, feel free to tell everyone details of your work and pay!" - so they are concerned they aren't supposed to answer (while many Universities publish salary data as part of open records laws, private industry routinely hides this info)
point-blank asking someone something that they might not feel comfortable asking is often considered rude or excessively direct, especially if you do not show in some way you understand you are asking for something potentially sensitive, and especially if you don't give them a way to save face in declining to answer


To have better luck, I suggest a script something like this:


Setup why you are interested in more details. "I'm going to have to be evaluating offers soon, which is so tricky when you don't know for sure what to expect."
Ask in a softer, less direct way, that makes it clear you understand if they don't want to answer fully. "If you wouldn't mind, could you tell me about your offer?" This sets them up to be vague if they don't want to give details, like saying, "oh yes, XYC Corp is very generous" or "it certainly will be nice not to be on a stipend anymore!" Not helpful, but it puts people at ease.
Some people will just tell you out right, while some will be circumspect. If it's really information you want, you can push a little, with something like a little conspiratorial lean in, and a more direct "just between you and me...can I ask you what the pay is?"
If they tell you, thank them for helping you with valuable information, as this part of the process is hard and often vague. If they don't tell you, it's generally safe to reinforce you understand and 'let them off the hook' with a "totally understand, I don't want to pry". If they let you in on inside information, don't go blabbing about it to everyone, and keep your sources to yourself. 
If someone asks you later for second-hand knowledge, be respectful of the fact that not all information is meant to be publicly available. If you aren't sure that you should yourself pass on the information, you can generally safely give a referral. "Oh I'm not sure, but so and so started working there, so you might ask them." Networking.


In closing, I will note that you are in fact correct that this is a cultural thing, and the norms vary. In the US business world it is common to treat salary info like something akin to a state secret, even though some are fine with talking about it openly and directly. I'm told that in east-Asian countries it's often the opposite, where it is commonly to openly discuss matters of pay as it's not considered so private. 

As you go you just have to be aware of how people react to things and how people seem to act, and practice finding ways to navigate such varying cultures, as even workplace-to-workplace things can be very different. General rules of diplomacy and tact still hold, they just take practice and awareness to develop and apply as needed.