We were having trouble for weeks with our pipelines randonly failing on bitbucket. The issue turned out to be with the /bin/bash --login command which is necessary for many node and ruby version managment systems and I wanted to put this out there for any one else that may be running into this issue.
This issue can be easily fixed by taking the contents of your .bashrc file and running those command directly in your bitbucket-pipelines.yml file and then removing the /bin/bash --login command.