Why is a "splash screen" so called?

Solution 1:

While the image certainly splashes all over the screen, the immediate origins of the term might lie in the comic industry. A splash page or splash panel

... sometimes referred to simply as a "splash," is a full-page drawing in a comic book. A splash page is often used as the first page of a story, and includes the title and credits.

A computer application's splash screen serves a similar purpose and was probably first introduced some time in the mid-90s. The comics terminology predates the software variant by a number of decades, possibly all the way back to before WW2. One source notes:

Eisner's contributions seem more valuable than ever -- but it would be a grave mistake to dismiss his work merely as academically significant. Eisner is a first-rate draftsman and consummate storyteller whose work is as passionate as it is personal. His Spirit splash pages are legendary for their intricate detail and gorgeous symbolism; in fact, the term "splash page" was coined as a tribute to Eisner, who often utilized the movement of water to convey a sense of urgency and motion to his full-page artwork.

One can only be a little sceptical about the claim that the term was coined as a tribute to Eisner as it was in use well before his time, in the newspaper industry. ODO's entry for splash includes the following definitions:

2 [with object] print (a story or photograph, especially a sensational one) in a prominent place in a newspaper or magazine:
the story was splashed across the front pages

2 informal a prominent or sensational news feature or story:
a front-page splash

From some time in the 1910s:

It was first printed in the " Daily News", but the "Cologne Gazette" seized it as a useful article for heartening Germany and borrowing a custom from this country, dressed it up with suitable headlines for a "splash" page. One of these headlines ...

From The Making of Modern Journalism (1927):

Page 5 of the Daily Mail was therefore what is now often called the main splash page, though the type used for the headings was smaller than that employed by morning papers of the present day, whether serious or popular, and there were no ...

ODO's entry also includes a note on the phrase "make a splash" which sums things up nicely:

make a splash
informal attract a great deal of attention.

A computer's splash screen (be it during boot-up, operating system start-up, or application start-up) does something similar by arresting the attention of the user with a pretty graphic, a bold title, and other miscellany while the application is loaded in the background. It is literally and figuratively making a splash on your screen.

Solution 2:

A splash screen completely covers what was shown before: it splashes content over the whole screen. The term has been in use since at least 1984, and originates from Apple Mac applications.

1998: FOLDOC

Here's a definition and origin from FOLDOC:

An initial screen displayed by interactive software, usually containing a logo, version information, author credits and/or a copyright notice.

The term originated among Macintosh users and spread, the synonym banner was once also used.

[Jargon File]

(1998-04-28)

1993: Jargon File

The earliest Jargon File defining the term is version 2.9.12 of 10 May 1993:

:splash screen: [Mac] n. Syn. {banner}, sense 3.

...

:banner: n. 1. The title page added to printouts by most print spoolers (see {spool}). Typically includes user or account ID information in very large character-graphics capitals. Also called a 'burst page', because it indicates where to burst (tear apart) fanfold paper to separate one user's printout from the next. 2. A similar printout generated (typically on multiple pages of fan-fold paper) from user-specified text, e.g., by a program such as UNIX's 'banner({1,6})'. 3. On interactive software, a first screen containing a logo and/or author credits and/or a copyright notice.

1989: Usenet

The earliest example I found in Usenet is from 9 July 1989 in comp.sys.apple:

The splash screen from SuperPatch 5.0 contains a notice that it is NOT to be uploaded to any information service, so there is no reason to wait for it to appear here, or on your local BBS.

1984: Apple advice

I found an earlier example in Google Books, in Jonathan Price's How to Write a Computer Manual: A Handbook of Software Documentation of January 1984:

This is also Apple-centric, as according to Web Writing That Works: "Jonathan helped Apple develop its user-friendly style (documented in his book How to Write a Computer Manual, revised as How to Communicate Technical Information)". It must have already been an established term, but clearly this piece of advice didn't catch on.

Solution 3:

In English(UK), a splash screen or splash back is a piece of glass or metal covering a section behind the cooker/hob. This screen provides an easy clean surface from which splashes of food -from cooking- can be removed without damaging / staining the actual wall covering(wall paper / tiles / etc).

This metaphor is carried across to software, in that the splash screen is a metaphor to show the loading software as being on a separate visual layer than the existing desktop or software. If this weren't the case, you would not know that the software was loading.