They forced their selves in to our universe from a neighboring ficton. The exact mechanism of how is yet unknown. We can assume the why was that they were interested in leaving their home ficton.
Goal: Clear, Concise and WittyI'd guess that the new, reason-specific thump messages were added to help deal with the main flaw of post-thumping, i.e., that removing posts without providing an explanation doesn't give those who see it after the fact a good indication of what kind of behaviors are considered unacceptable.
Heapers’ HangoutThe special thumps are, as Ironeye said, reason specific. Which means they don't get used nearly as much as the general thump.
There are like 4 others. Reason specific ones let tropers know why that message was thumped.
You got some dirt on you. Here's some more!

How did they come about?