The phrase "holding the bag" typically refers to being left with responsibility for something that someone else has caused or abandoned. If you are looking for synonyms for this phrase, you might consider alternatives such as "left in the lurch," "picked up the pieces," "bearing the burden," or even "left holding the baby." Each of these phrases suggests some level of unexpected or unwelcome responsibility for a situation, and all of them imply that the person responsible is not present or has absconded in some way. Choosing the right synonym will depend on the context of the situation and the level of emphasis you want to place on the unexpected nature of the responsibility.