Objects snapping together

Last time I did any modelling was at the beginning of 2018, but then I started Uni and my time evaporated. Now I have some time off, I’m getting back into it but I’m having an issue that I cannot find the answer to on Google. Also, I get the feeling it’s an incredibly simple issue and fix; please be gentle!

https://imgur.com/d57V3Ez

I’m trying to create a set of teeth (part of a whole character, as seen above, but when I box select all the vertices at the bottom and attempt to move them down, the gap between the front two teeth snaps together (as seen below).

https://imgur.com/OOoZSYQ

How to I stop this from happening or/and how to I fix it?

Try disabling clipping temporarily on the mirror modifier.

I knew it’d be something basic. Thank you very much for the assist.