In several programming languages ; is required at the end of every statement. Its usually seen at the end of every line but multiple statements can be put on one line separated only by semicolons.
As it originated from a tech literate crowd I suspect the semicolon has been added to separate the two statements
tl;dr - programmers like using semicolons to separate statements
edit - A citation has been requested, I originally made this statement based on personal experience of first encountering it only amongst a technically proficient audience before seeing its usage spread.
Unfortunately being internet slang its a touch difficult to track down a verifiable origin, however.
KnowYourMeme's article on tl;dr states
It’s not entirely clear
where and when t;dr first began, but
the term has been used on Genmay since
at least 2003. Example: On June 19th,
2003, gen[m]ay user waptang created
the thread tl;dr asking “what does it
as per this article genmay (General Mayhem) is a spinoff forum from [H]ard|OCP (Hardware Overclockers Comparison Page).
Overclocking is a reasonably technically involved process, so I'd feel safe calling them a tech literate crowd.
However, essentially this is conjecture on my part.
Commenters on a reddit thread on this subject suggests a different theory
They are two separate clauses of a
related nature. It is correct grammar
correct to place a semicolon between
them if you did not want them to be
but more than that, it shows that the second sentence is a result of the first.
"it was too long therefore i didn't read it"
Though I'm still inclined to side with a latter comment stating
Because it is the only semicolon most
of us get to use outside of
TL;DR - Because
TL;DR pt 2 - I believe this to be the case however ymmv