The commentaters explain that they "swept up" the lost items that had been left behind as well as all the stragglers who fell behind in the march. If anyone lost an item, they went to the banner of Dan and were able to retrieve it. Rabbi Silber in his drasha this Shabbos explained that there are two reasons that the tribe of Dan was chosen for this position. One was that as the most populous tribe, they were able to spread out along the line of march and ensure that no-one was left behind and that nothing was lost.
ונסע דגל מחנה בני דן מאסף לכל המחנת לצבאתם
Then journeyed the banner of the camp of the children of Dan, the collector of all the camps according to their ranks.
The alternate explanation is that the tribe was spiritually weak and actually smuggled idols out of Egypt. This is also shown during the incident of פסל מיכה in Judges. The tribe of Dan in their journey to the north swept up an idol and used it to worship Hashem. Similarly, Yerav'am set up one of the idols in the territory of Dan to ensure that the 10 tribes would not reunite with the tribe of Judah. The other tribes would not have allowed these idols within their territories. In order to ensure that they were able to merit the rewards of following the Torah, Hashem gave them the significant mitzvah of השבת עבידה.
The significant point is that the word used has the connotation of gathering or "sweeping up"
Shortly afterwards (11:4) we have the complaints about the manna and the crying for meat.
Here again we see the root אספ so that we have the connotation of "those who had been swept up". That is it was those who left Egypt not because they wanted to follow Hashem or get the Torah, but because they had been swept up in the enthusiasm of moment. Notice that they were not overcome by a desire or or actually lacked nourishment, but they deliberately cultivated a desire. If they had been given lamb, they would have complained that they did not have beef. In any event, they had brought out vast flocks of sheep and cattle from Egypt. The nostalgia for fish (which only the poorest of the poor would be given) shows this. They deliberately chose to "remember" something that a dessert environment would not have.
והאספסף אשר בקרבו התאוו תאוה
And the rabble that was among them cultivated a craving.
The commentators state that this problem first arose with the ערב רב ("mixed multitude"), those slaves who came along with the Jews because they saw that Hashem was stronger and they wanted to get out. However, they did not want to serve Hashem nor did they want to follow the standard of morality required by עם קדוש (holy nation). Similarly, they knew that when the people came into the land, they would have to earn their own living and could not continue to be supported by the masters. Even in the dessert, the "free food" was restricted in that it had to be gathered by the individual and was not handed to them. The medrash say that the ease of gathering was directly proportional to the level of spirituality. These people were on the lowest level even though they were not as low as the 80% who Rashi says died in Egypt.
We learn from this that those who work and strive to act properly are rewarded by being able to improve themselves. On the other hand, those who merely allow themselves to be "swept up" by the enthusiasm of the moment (no matter how good the cause) are just as liable to be swept up in the mob enthusiasm for an improper cause. We can see this in modern day politics as well.