Why Does the Moon Have Quakes if It Has No Active Volcanoes?

I understand that Earth’s earthquakes are often linked to moving tectonic plates, but the Moon doesn’t have the same active surface geology. So what causes the shaking detected there?

I’ve come across the term lunar seismic activity in discussions of Apollo-era instruments, but I’m unclear whether moonquakes are caused by internal heat, impacts, or something else. How do scientists tell those possibilities apart from the signals they measure?