Eclipse will rebuild all projects if it detects that the project structure has changed. This is particularly apparent with RealView Profiler. When a new profiling and analysis file is generated it is added to the current project – Eclipse detects this and determines that the project requires rebuilding.

You can disable this behaviour:
Go to Window --> Preferences
Expand the Run/Debug tree and select Launching
Under General Options uncheck “Build (if required) before launching”.

Note that the project will need to be manually rebuilt before each launch if changes have been made.