This is actually easier than you think, and represents a major shortcoming of our modern civilization - specialization.
First and foremost, it's safe to say that all traces of human civilization on Earth are now well and truly gone. In other words, no one is going to find caches of technology or resources just lying around.
Second, consider that the skills necessary to survive on a wild, and dangerous planet are not the same skills needed to survive on a generation ship. Those people were probably raised with certain societal roles in mind. Societal roles which corresponded to the needs and requirements of a generation ship. And suddenly, you thrust these very ignorant folks into a wild situation.
But Andrei, you say, surely they have libraries of materials that they can read up on! Sure they do. They probably have the boy scout's guide in a handy PDF file. However, the Earth's changed in the 4.8 million years that humanity has been gone. That information is not only stale, unexpected situations are bound to occur. Those texts will be poor substitutes for actual experience.
And acquiring that experience / revising their survival knowledge is probably going to cost them dearly:
Ignorant human: Oh, look! A pretty insect! touches it and promptly drops dead because it was poisonous, and didn't exist when their survival book was written
But that's not necessarily a reason for them to devolve to cave-man status. There's a few ways (which can be used in conjunction) in which this might come about though:
1) Ship crashes
Upon nearing Earth, it's quite possible that their ship suffer a major malfunction, and that it crashes. A lot of the crew might survive due to lifepods, etc., however not a lot of survival gear and equipment would be salvaged, and the survivors might also be scattered across the globe.
This is, really, the easiest way to explain it. Ignorant, poorly prepared people with little to no resources ... The survivors would discard their civilized sensitivities rather quickly.
2) A few key people die / resources are lost
We all know how to use a cell phone. But how many people know how one actually works at a hardware level? How about build one?
If a few key people die, a lot of knowledge and equipment might suddenly become inoperable.
If you want to max out the irony index of your story you might have the survivors lose security access to key resource vaults when their leader dies in a really silly accident.
Say that their library of survival texts, and maintenance manuals was lost in a fire. The existing techs would teach their skills and knowledge to some apprentices. But with each passing generation some information would be lost, until eventually that technology seems as magic, and finally just gives out due to lack of spare parts, and adequate maintenance.
If the knowledge loss is dramatic enough (techs die trying to fight the fire which consumed the library) then the situation would deteriorate much, much faster. This would also happen if a key resource - say, a nano-tech creation engine used to manufacture all spare parts and equipment - suddenly malfunctions and is simply not fixed before essential resources start running out. In that state of panic, and with a few emergencies thrown in, not to mention some poor leadership, they would be doomed.
Next thing you know, perfectly civilized human beings are killing each other for food, and all pretenses of civilization are out the window. Read Lord of the Flies by William Golding for his take on how a group of civilized school children stranded on an island devolve to barbarity.