Strange vertice behaviour

Hello, instead of explaining my problem I just show you:

Does someone know what can be causing that?
Thanks

Why have you failed to supply a link to a .blend file.

The images miss out 95% of the blender interface so we know zero about your object or settings you are using !!

Sorry I thought it could be a common problem, like a bad shortcut manipulation or else, it happens often to me…
Here’s my .blend
character14.blend (939 KB)

Thanks for your help

On Options tab, switch off Topology Mirror option.

Thank you!