AITA for waiting in line at the grocery store while my parents finished shopping?

So I went to the grocery store today and the lines to pay were a bit crowded, so my parents asked me to already wait in line while they finished grabbing the last couple of items. We usually do this if there’s a lot of people. If it’s my turn to pay and they aren’t there yet I just let the person behind me pass, so I don’t stop the flow (for obvious reasons). Even though that wasn’t the case today, this afternoon another customer was really angry because of this, claiming it was unfair and disrespectful to others that were waiting in line too, since "you can only wait in line once you finished shopping, not before that". I have no idea what’s the logic behind his point of view, and he didn’t even explain it to me then because of how frustrated he was. Is this a cultural thing? Am I wrong here? As far as I know, as a customer you should abide by the rules of the establishment, and the grocery store has no rule against doing this. I didn’t prevent other customers or the cashier from continuing their businesses, and I had to wait in line like everyone else… So, AITA here? If so, please let me know your logic in detail, I’m curious to know if I’m missing anything :3

Update: I need to clarify something. My apologies if I expressed myself wrong, English isn’t my native language. When I mean couple of items I’m talking about dog food only, the big bag that’s heavy and a small bag with the little treats of different flavors. Everything on the same aile. We forgot to pick it up when we waited in line, and they had to take the shopping cart because it was too heavy to carry all of it by hand from that aile back to the cashier. We didn’t think we had to go to the back of the line for forgetting one item that required the shopping cart to carry, that’s why I was left there waiting.
Shout out to Betalisa and the other redditers that suggested I should add this info as context!