Why do some Catholics file up for commmunion? My wife was telling me about how in Spain, it's common practice to just go up to receive communion pretty much at random whereas in my experience I've always filed up (even when I was in Germany for World Youth Day). So I'd imagine some Catholics file up and some go up at random.  Buy what is the reason for filing up, or if that is indeed the more ancient practice (which I somehow doubt it is) what is the reason for going up willy nillly?
I can give you an answer based on my experience.

I live in Rome, I'm Catholic, and I have travelled quite a lot. Every time I travel I search a local catholic mass.

I can tell you with a 99% of certainty that this is a cultural matter, dependinding on the community. 

Here in Rome people is more or less ordered and goes in line. In France they have a more complicated 'algorithm': the one who are in the last benches go first, then the line builds up bench by bench from the last to the first.

A convincing example is this one: I went to the masses for holy week in Amsterdam. That is to say 3 masses: Thursday, Friday, Sunday (Saturday there's no mass in the holy week). As Amsterdam is quite multi-cultural, once I went to the local (Dutch) community mass, once to the french speaking community mass, once to the Spanish community mass.

In the dutch version they went in a line starting from last bench. In the french-speaking, mostly were from Africa and went in a line which was randomly formed. In the spanish community mass, people was much more 'unordered' and they crowded in the direction of the priest giving Eucharist.

So, summarizing, It seems really to be a cultural matter.