Thomas Foster, in his book How to Read Literature Like a Professor, has this to say about symbols:
“Here’s the problem with symbols: people expect them to mean something. Not just any something, something in particular. Exactly. Maximum. You know what? It doesn’t work like that… so some symbols do have a relatively limited range of meanings, but in general a symbol can’t be reduced to standing for only one thing”(Foster 97-98).
Symbols, therefore, can stand for many ideas so one must look broadly at the all possible connotations of the word "Flight" in order to consider all possible ideas expressed by Steinbeck. In the analysis of "Flight" it is explained that
The central idea of "Flight" is Pepe's transition from boy to man. In the course of running from his crime, Pepe starts as a youth fleeing responsibility. As he loses the tools that define his humanity, he is reduced to crawling on the ground like an animal, wriggling like a snake and "worming" his way along.
As suggested above, flight "can't be reduced for standing for one thing" as Foster claims. Flight can be fleeing from danger and fast travel. However, flight can suggest freedom but can easily be delayed. Flight can be a frenzied escape, but it also can be slow and painful depending on strength. The symbol of "flight" changes from the beginning of the novel to the end, which is symbolic in itself. Read more about significance of the title "Flight" in the analysis of theme.
I think it also might be called flight because it mentions several times about the flying eagle and he looked up towards the sky a lot and i think the eagle could represent hope because whenever it was mentioned Pepe gained the strength to keep going. Possibly it might be a symbol of his father?
I also think it probably has a lot to do with fleeing, seeing as that is what he did.