The main intention behind the bounties is to reward answers that have done something original. They include, but are not limited to, posts that have lead to new packages or libraries. It is true that most of the bounties were awarded to TikZ posts, and the reason is as simple as the fact that I am a bit more comfortable judging for those whether or not they are a major step forward than for other tags. Also I "earned" most of my points with TikZ related posts, so I just pass these points to other TikZ posts, and IMHO this does not "take away" anything from other tags. Altogether I do not understand at all why one should not be allowed to do that.
P.S. As for the "why now" question: true, the main motivation is that the points will decay once my account is deleted, which was scheduled for August 3. Why would it be "better" to let the points decay than to pass them to others? Let me mention that it is not the case that these are the first bounties I awarded. One bounty of 500 points I remember was to a user who wrote a stellar answer, but after she was done the OP changed the question. Unfortunately, the user seems to have become inactive, yet the discussion why this happened is off-topic here. Nevertheless I'd like to reiterate that the purpose of all these bounties is to acknowledge original codes. And I feel that if there was a general agreement that originality (as opposed to just copying) is something valuable, many problems that surface here and there on meta could be avoided (and the current discussion would never have emerged). But this is just an opinion.