What is the background/origin of your legal knowledgebase
For the most part, my background developed from reading case law. Court opinions are most useful for acquainting oneself with legal concepts, doctrines, and argumentation.
A word of caution: Court opinions are not for ascertaining the facts that led to a legal dispute. Depending on their bias, judges can (and do) cherry-pick evidence just to make their decisions look as "factually based". To have a more reliable notion of the facts of a case, one needs to see the actual evidence. At most, court opinions purport to quote the allegedly decisive evidence (usually bits and pieces thereof).
why do you participate on stack exchange?
To help the audience attain self-sufficiency on the legal matters they encounter.
In general, knowledge is power. It helps people become less vulnerable to others' ineptitude and/or misconduct. By "others" I mean not only lawyers, but also physicians, employers, car mechanics, financial advisors, politicians, and so forth. However, law and health are by far the two fields where the consequences of ignorance and blind reliance on a licensed charlatan tend to be most detrimental.
By developing enough awareness of the law, the public would have elements as well as the determination to scrutinize judges and lawyers. In turn, that scrutiny would have dissuasive effect on some of the rampant corruption that goes on in the judiciary's lucrative business.
Is it about helping others?
To a great extent, yes. That is why I dislike when under pretext of "asking for legal advice is off-topic" others close interesting posts about legal, real-life matters. By closing the posts, those users prevent me --and others-- from helping the OP understand his legal issue and work his way out of it.
Do you appreciate the added factor of entropy from letting your research topics be decided by real people's questions and interests?
Not exactly. Others' questions and interests oftentimes are thought-provoking and might prompt me to do some research, but saying that they decide it would be an overstatement.
Other topics such as copyright issues or some politician's latest occurrence on TV are quite recurrent on LawSE, yet I hardly ever am inclined to delve in them.
most interested in some of the top contributors who seem to have an insanely broad (as well as deep) base of knowledge
I cannot consider myself in that category. Nevertheless, the background I have obtained enables me to make a preliminary conjecture [about issues] which upon research oftentimes turns out to be substantially accurate.
Also note that scoring high on Law SE (or any SE site) does not guarantee that the person is that knowledgeable. I am not supposed to single-out anyone, but the truth is that the audience's upvoting is often influenced only on whether the author has a high score even if his answer is devoid of sources or accuracy.