I saw this a few times when my ping spiked past 150 ms or something like that. My theory is that the server has a timer that says when the light attack animation changes to a medium/heavy weave. Due to this timer being exceeded due to excessive latency and/or packet loss, the server misidentifies your light attack weave as heavy attack. Mobs falling down after a delay is simply the server killing them with the misidentified heavy attack, then the information feed back to the client, although you only see the light attack animation. The death animation delay is about as long as the heavy attack animation would have taken, around ~1.2-1.4s.