Why would an incoming programmer rename all properties in a javascript class with double underscore? [closed]

I’ve been working on a javascript project (webgl / three.js stuff) for some time now.

Recently a few programmers were hired to write the rest of the site as a react web app – and to add my content to that app – while I was relatively absent.

They incorporated my main class (the top level of all the code having to deal with the 3d, so to speak) into a class they called ViewAPI.

My question: they renamed many properties and methods of that class, adding a ‘__’ or ‘double-underscore’ prefix.

Is this some established convention? Other than perhaps with the intent of name-mangling – what would might have been their reason to do this?

Follow-up: I found it a little troublesome to adopt a new naming convention on the project I’d been working on for so long. Would their changes be considered appropriate?