All-caps indicated importance and emphasis for thousands of years centuries, and were occasionally used to indicate shouting as far back as the 1940's (as evidenced in Philippa Schuyler's biography).

The practice became widespread in Usenet newsgroups around 1984, when one user summed it up as:

there seem to be some conventions developing in the use of various emphasizers. There are three kinds of emphasis in use, in order of popularity:

1. using CAPITAL LETTERS to make words look "louder",2) using asterisks to put sparklers around emphasized words, and 3) s p a c i n g words o u t, possibly accompanied by 1) or 2).

and

if it's in caps i'm trying to YELL!

The source is the New Republic, but they cite their contributing editor John McWhorter who's a well known linguist.

http://www.newrepublic.com/article/117390/netiquette-capitalization-how-caps-became-code-yelling