Can applying for PIP trigger ESA review?

Hello lovely people, I'm a little bit puzzled this morning so I hope someone here can help.

I've been on ESA (IR, support group) for a number of years and sent back my last ESA50 (I think?) in June 2019 - heard nothing after that, money kept appearing in my account so I thought nothing of it.

This year, I decided to apply for PIP as well - I've been putting it off but with the help of the guides here and knowing that I'd be getting a telephone assessment, I managed that horrendously long form. Got that in August, DWP confirmed receipt in September and then two weeks ago, I got a letter informing me of a telephone assessment which happened today.

All went well, person on the other end was pleasant enough but some of the questions weren't quite what I'd prepped for the night before but I got through them and at the end they said that they had enough information & all would be well. I thanked them for their time and said I wish I had applied for PIP sooner... which is when they said that this was nothing to do with PIP but was in fact an ESA review.

I asked them to check if that was correct as it had been so long since I'd sent the ESA form back (I knew it was last year, they confirmed it was June 2019) and while they said the timing was a little odd, obviously they couldn't comment on anything PIP related.

Is this a normal thing to happen? Usually I've had ESA assessments within a few months of them getting the form so I didn't even consider it could be for that. Plus I was hoping that everything PIP related would be over and done with today, so now I'm wondering how long that's going to take - at least I've got all my interview prep done for that!