Chunking and learning vocabulary Learning vocabulary seems to be efficient if the words are learned in a fixed sequence, but it seems to be harder to recall these words in a different sequence then the learning sequence.

I wonder which theory can explain this effect, is it "chunking"? 

Some definitions define chunking as a process where facts are organized in groups "chunks" that can be better memorized then the single facts.

In other definitions, it is described as an effect, where the elements of a chunk can be better predicted, because the probability of the occurrence of an element following another element is higher. Or in other words, we learn, that in the context of a special chunk an 'a' is mostly followed by an 'b' and so we can memorize the chunk 'ab' better.
I think you've got a reasonable handle on why learning vocabulary in a single, fixed order is probably not a great idea. You may also be interested in what is called context-dependent memory, where you recall things best in a context similar to that which you learned those things in (shitty sentence, but you know what I mean).