You have to consider that people start up their own pug groups for different reasons. I can see two categories here. What they have in common is that the leader doesn't mind the slight hassle that may occur in forming a pug, especially for a pug raid.
The first category is someone starting their pug for reasons such as that they don't find anything they want to do in the LFM list, or they don't have a network of friends available to run with (at that time) and third, they are already grouped and would need some more people (this third reason has almost died due to various reasons and changes, except for raids). Some people also like to start and lead groups because they think it's fun to lead, rather than out of necessity.
Then we have the second category of group-formers. Some start their own groups because it can be a little power-trip for someone with a low self esteem. You get to pick and choose people and you get a little bit of attention. There is also the possibility that they do not get accepted into other player's pugs due to various reasons (such as personality or for example the combo of "low & loud" game knowledge), and thus have no other option than forming their own.
The second category you can sometimes spot, with either macho or illogical LFM's, but of course not always.