Just ask him

For me, I don't email anymore unless it's super important, I just got sick of how much miscommunication could happen with emails. Anyway, I used to email weekly... and during that time, he was awesome with replies and on occasion would do something like this too and I agree, it felt dismissive so I called him on it, after that, he never did it again. He said he would only reply when he had time to properly reply, it was nice. So yes talking to him is best