The thoughts that came to me when reading this are similar to the others here. Perhaps you felt things building up, (you used the word stacked ), this could be stresses, clutter or a combination. The boxes represented diff issues or objects. They were STACKING up! When do we feel pressure the most on our minds?....when we rest or lay down. The three men may be your guides. They may have brought you the grocery cart as a symbol to remove the stuff out. In other words here’s a cart get cleaning. Unpack the stuff. You may have been over burdening yourself and they are letting you know.
Just a fun analysis. Do with it what you will. I think the fact it stood out to you is a clue to its importance.