What a great/awful question :)

Great, because it allows me to bust out some real historical digging and research and awful because I'm going to be completely unable to directly answer it! In fact, you may have broken this sub! The only way to attempt an answer is the trifecta of rule breaking- "I don't know, but here's some anecdotal evidence and some guesses based on vaguely related sources".

Hear me out, mods!

First, a little bit of quick history on Titanic's return trip. She would have sailed from New York on Saturday the 20th, stopping at Plymouth rather than Queenstown and then back to Southampton. From there, like with Olympic, she would have settled into a steady run of Wednesday departures from England and Saturday departures from New York.

The passenger list for her return is gone, we just have no record of it (that I've seen. If one has popped up recently, I haven't heard about it). What we do have is a few little anecdotal notes- first class was booked pretty full, New York Governor Dix was supposed to sail on her. We know a few people who were given alternate passage. Last Titanic survivor Millvina Dean went home on the Adriatic. RMS Lapland left New York in place of Titanic, transporting both passengers and crew back to Europe, as well as all the mail Titanic was supposed to carry. She was held up at the last minute, as Senator Smith stopped her sailing to remove 5 crew who had been summoned to give testimony at the Senate Inquiry. What crew were left, disembarked at Plymouth and were sequestered pending questioning.

And none of that answers your question. So, here's where we have to start digging around to make assumptions. Remember, Titanic- in the grand scheme of things- wasn't all that special. She was for the very brief time she sailed of course, but that was going to be short lived. Her reputation as the largest, most luxurious (depending on who you ask), moving object would last roughly a year, after which she would be overtaken in all regards and become just another ocean liner on the busy Atlantic run.

So, it's fair, I think, to look at other ocean liners on that busy Atlantic run (some of whom were at one time the most famous ship in the world), and see who they carried and why on the eastward journey.

The first problem we run into is twofold. One, I think it's really important we look at pre-war, as it's natural that both immigrations and reasons for traveling probably would be hard to honestly compare between peace time and world conflict (troop transport alone). Two, if we do this- what sources we have, that is, passenger lists, don't help us a lot as they are mostly first and second class- not third. There'd be very little reason to keep third class passenger lists once they passed immigration, and even less reason to keep them for a voyage that didn't happen.

We also need to understand the history of third class travel. Not too long before Titanic, in an attempt to stem the massive exodus from Europe, governments were publishing pamphlets describing the horrors of Atlantic travel for the working class, many with sketches showing the 'horrors', that such travelers had to 'endure'. From an 1884 edition of The Graphic-

In these days, when the excitement of emigration, which is draining our own country, England, Ireland, and many other countries on the Continent, of their best blood, is at its height, it may not be the kind of place to bring under the attention of those of your readers in whom the desire to emigrate may exist my own experiences, in common with many others, in crossing the "Herring Pond," on board a large Ocean Liner.

To be forewarned is to be forearmed, and perhaps from the few remarks that I shall make, a hint or two may be gathered which may hereafter turn out to be beneficial to some.

This was still fresh by 1912, as one of the OCL'S selling points was how comfortable and luxurious third class was- 20-30 years previously it would have been first class! Third Class went from having to provide their own food and bedding to having bunks, stewards, chefs providing three meals a day plus afters, common rooms, smoking rooms, heaters, a piano!

By the 20's, third class was being marketed as the economic way to travel. All the same amenities as first or second, just less grandiose. Essentially- coach on a plane today. Not ostentatious or luxurious but still comfortable with impeccable service.

So, now with that context, let's see what we can look at try and answer your question. While passenger lists pretty much don't exist, immigration records still do and are available in the Board of Trade section of the British National Archives. I pulled a sampling from famous, contemporary, ocean liners that sailed around Titanic.

RMS Olympic- Return voyage from New York, July 1911.

Nationalities: Polish, Hungarian, Italian, German, Mexican, Chinese

Occupations listed: Laborer's- their wives and children, barbers, reverends, students. Very difficult to read but this is a sample.

RMS Adriatic: Return voyage from New York, July 1911.

Nationalities: This is a list of returning British nationals.

Occupations: Domestics, housewives, gardeners, students, hull workers, merchants, coach driver, engineers, clerk, ministers.

RMS Mauretania: Return voyage from New York, August 1910.

Nationalities: Russian, Austrian, Sweden, Hungarian, Syrian, Serbian, Belgian, Spanish,

Occupations: Laborer, tailors, carpenters, painters, cooks, domestics, farmers, teachers, wives, and children.

RMS Lusitania- Return voyage from New York, September 1910.

Nationalities: Americans,Syrians, others listed above.

Occupations: Waiters, Librarians, Miner, Musician, Clerks, etc etc-

Now, this is a very small sampling trying to paint a larger picture. What I think it shows us, however, is interesting. The third class weren't all poor, many were what we would consider middle class- teachers, clerks, chose to sail third when they could have probably afforded a small second class berth, as many did on Titanic. This is smart economically- if you were sailing alone, it would be smarter to book a single berth then a whole cabin- especially for one of the more popular entries across the records- students. We start to see this migration down a class as third class became the second or even first class equivalent on older steamers.

What we aren't seeing is the overwhelming amount of tradesman and laborers who boarded Titanic. The return voyages seem to be MUCH more varied, and casual, filled with travelers looking for economy not just necessity.

The other thing to consider is that ocean liners very rarely travelled full. At the height of the summer, absolutely, but rarely that equivalent. Titanic wasn't even close to full, even with passengers from other ships transferred onto her. Since we have no surviving return passenger manifest, we have to rely on hearsay. We have one note from an unidentified steward speaking of a lovely quiet voyage, but that he heard the return was much more full. So, hearsay from an unidentified source... not great.

Titanic was sailing in April, a very slow season so her lack of passengers wasn't surprising. If the account is true, I would wager a guess that the third class berths would be those returning for/from vacation, rather than immigration. The end of April would have been the very beginning of the travel season, and the rough coincidences with spring break and the end of easter break (April 7th that year) may have explained the influx of students.

The picture these samples paint appear, to me at least, to be travelers and vacationers- not immigrants. Middle class employees with steady, committed jobs are very often found on contemporary records, as opposed to general laborers and tradesman who would travel for opportunity.

Now, that of course, is a guess. It would be impossible to know why all these people were individually traveling, and why, potentially- the return voyages would be busier. We just dont have the records to make an honest study here, they simply don't exist. Therefore, we can't sit down and compare the ebbs and flows of passenger counts across years and seasons.

So my answer is a guess, and it's a guess based on what few scant contemporary records we have of Titanic's fellow ocean liners. As there's no reason to believe Titanic was any more special than the others, it makes sense that those who boarded her boarded her because of her scheduling, not because of her fame.

What I would do, is hope that someone more knowledgeable in pre world war 1 immigration- specifically to Europe - will chime in and explain more. As far as Titanic specifically goes- the information just doesn't exist anymore.