PS: I am shocked by this activity. Even during the communism revolution, the government has left the university intact. The HK policy lost their mind...
The real aim was probably the students. Public perception, especially among the mainland camp, is that this is mostly a youth protest (but from what I've heard, it's far from solely a youth protest). Back in August, they were all saying that the protests would die down and fizzle out once the semester started (which obviously didn't end up happening). In their eyes, the educated youth are the ones "driving" the protests. In their eyes, taking and suppressing the university would be a strike at the core of the pro-HK movement. That didn't work, and now their move is backfiring into further publicity and support for Hong Kong.
So I suspect it's all interior forces, not PLA.