I'm surprised that people are being so cynical here. I can easily picture all the food running out. All it takes is several hungry teenagers or people in their 20s to serve themselves first, and there is very little left for everyone else!
Or, as others have suggested, people were taking their food and not moving back from the table. In which case, this couple is darned if they do, darned if they don't. If they hang back, they're shy and passive-aggressive. If they try to push through the crowd, they'd probably be deemed "rude" (I can actually picture etiquette advice saying 'If no one is letting you through to the food, the polite thing to do is leave early and eat elsewhere, rather than physically jostle people out of the way').
At any rate, I believe this couple should tell the hostess the truth, rather than make up some excuse.