I think its due to some recent changes in ADT 21.1.0. Anyhow, to resolve this, you may configure the following in Eclipse:
Window -> Preferences -> Run/Debug ->
Launching -> (under Launch Operations) Always launch the
previously launched application
In case you want to open another project, then you need to revert this process as it will always launch the previously launched application.
Also, keep in mind that this issue may be fixed in the future versions of ADT, so don’t rely hard on this solution.