So just going to reply to the topic subject, so forgive me if this response was already handed out. Also feel free to point out things that aren't correct, as this is mostly just my own personal conclusion:
Fact is, the reason humans are a productive species who gravitate towards creating functional societies is because we're given this incentive thanks to endorphins. With a proper upbringing, this is what lets us not only define 'Right and Wrong', but also be able to try and shoot for 'Right'. That's why, most of the time, it feels good to do the right thing and it feels bad to do the wrong thing. It inherently inhibits us from harming other humans.
Now how does this relate to American (I'd actually say this is transcontinental if we peer at Germany and some Nordic nations) Liberals getting offended? Well, there's little better endorphin rush than being on the side of absolute righteousness. What better 'Right' thing is there to do than to fight against something that is unquestionably and irrefutably 'Evil'? That being said, Life doesn't deal in absolutes and, beyond hypotheticals, there's nothing that's truly evil or truly good. Sadly, your production of endorphin doesn't exactly account for that, which means you can just as easily delude yourself into thinking there IS an evil and fighting against it will provide you that rush just the same as somebody who's in the afterglow of taking down a serial killer or whatever extreme would suit your needs more. These people are defending a "weaker" type of people against a "strong" type of people. Just that simple.
So why offense? Because it's the easiest way to get that rush you want. What easier way to justify yourself if you can do it by sitting down screaming at the internet at the safety of your home? This kind of thing has existed for a long time, just under another name before our darling internet warriors came around (See; https://en.wikipedia.org/wiki/Slacktivism).
It's fast. Convenient. Gets you off. And your conscious is, yes completely delusional, but clear. Other demographics, be it race, ability, or age, are simply damsels in distress for these people to get their feelgoods going. Ironic, considering they loathe the notion.