See https://electronjs.org/blog/electron-2-semantic-boogaloo.

As of version 2.0.0, Electron will strictly adhere to Semantic Versioning.

Major Version Increments

* Chromium version updates

* Node.js major version updates

* Electron breaking API changes

Sounds a bit like angular and we might get electron 3 and 4 very soon if they feel a need to update Chromium or Node.

A major version bump is for introducing breaking API changes. That’s all it means.

If they're following semantic versioning, then they bump the major version number whenever they make breaking changes.