Visual Studio deploys entire solution when previewing a report

This has only happened a couple of times to me and I can't seem to reliably repeat it but, frankly, it's terrifying.

We have an SSRS 2014 reporting solution with about a dozen projects containing maybe 100-150 reports in total. I do some work on a report, go the preview, and Visual Studio deploys the whole thing straight up to the report server. NB, not just builds, deploys. Thankfully we tend to implement changes and deploy them straight away but, in theory, devs could have all sorts of untested changes in the solution that they aren't ready to deploy.

I've been working with SSRS for at least a decade and have never seen this behaviour before now so I'm pretty sure it's non-standard. My hunch is that it's caused by some setting or configuration in the project but I'm damned if I know what. Has anyone else seen this or know how to prevent it?

I should say that the way we're working here isn't particularly smart (no source control or formal deployment process) but I'm only here in a consultant capacity so can't directly effect change. If this is some configuration issue it's likely I can get that fixed. Getting them to change their processes to accommodate it is a lot less likely.