“from this time” = “upon this” (KJV) could also mean “for this reason.”

“back” = eis opiso = to the rear, or to the behind things. Could be translated “they went away to the things they left behind.” Godet sees this as “more than simple defection; they denote the return of these people to their ordinary occupations, which they had abandoned in order continuously to follow the Lord.”

Not liking what was said and not accepting its consequences was a temptation that resulted in rejecting Christ. Looking back ended in going back. When the recently released Israelites looked back (Num 11:4-6) they remembered the good things from their former way of life but completely forgot the overwhelmingly worse bad things, i.e. slavery, murder of their children, etc. When we look back, our memories become one-sided and we often choose the comfortable and lose the best. That is why God does not want us to look longingly back at our former way of life (Gen 19:26, Luke 9:62).

Num 11:4-6  The rabble with them began to crave other food, and again the Israelites started wailing and said, “If only we had meat to eat! We remember the fish we ate in Egypt at no cost—also the cucumbers, melons, leeks, onions and garlic. But now we have lost our appetite; we never see anything but this manna!”

Gen 19:26  But Lot’s wife looked back, and she became a pillar of salt.

Luke 9:62  Jesus replied, “No-one who puts his hand to the plough and looks back is fit for service in the kingdom of God.”

Jesus had sifted the true disciples “from the false, and the deep from the shallow.” (Morris 382) Whether they were following Him because they thought Him to be a rabbi who taught great things, or they believed Him to be the Messiah who would set up His earthly kingdom, or perhaps just for the sake of the material benefits of bread, fish, and healing which He had been providing, whatever their reasons, they now deserted Him. His claims had proven that he was not just a rabbi, He did not fit their expectations of the Messiah, and His demands upon them were too great for them to continue following, even for the material benefits associated with it. Following Him now proved to be something completely different from anything they had anticipated. Following Him was now impossible apart from a total commitment, which they were unwilling to make.