This is pretty standard because it is a quick and dirty proxy to those who have most the internal experience. Sometimes companies will layoff people who are more senior because of cost, but the reality is most senior people are not making double of those who just joined. A 20% difference in pay simply is not enough to take on the risk accidentally getting rid of people who have key knowledge.
I didn't know that was common sense but it's something I have personally experienced.
I joined the IT team at a major food industry, and I would look at the office floor and people would point me to those who also had just joined.
Things started to go sour an year later (cash related), and people slowly started to leave. On my last day, I looked at the office space again and something seemed interesting: everybody working had been at that company for 3+ years. All the newcomers had left. I mentioned this to a couple of people and asked them "do you see anyone here that has joined in the last 2-3 years?" and the answer was "Crap, no".
So it's not only management letting go of the last hires, but these hires also leaving (more of that, I suppose).
This is indeed a general rule, but there are oddball cases, like when the firm I was at was acquired and the new owners kept all the back-office staff, but laid-off the developers (the acquisition was for the customer base, not the technology).
Which raises an interesting question - what's the valuable parts of Twitter? Customer base, for sure. Architecture for scalable messaging, absolutely. The rest of it? Not sure.
I'd argue that Twitter's brand is among its most valuable assets. It's globally known. Used and promoted by influencers like celebrities, artists, politicians, media, business leaders, activists. It's all over music, TV, media of all types. Heck, an entire movie was made about it.
That sort of broad public awareness is very hard for any company to get.
