Random "waiting for next build executor" message

We have couple of multibranch pipelines and recently noticed that with every push/commit made in GIT repo, the build process would start for all branches instead of just master branch. This results in halting of all pipelines build with the message of “waiting for next build executor”. As a workaround, we abort all the builds and execute master branch build manually(which is suppose to happen automatically).
Is this something related with configuration error or version issue(2.280)?
Fairly new to Jenkins so not much experience, kindly advise.