Shrug. Who knows. He could have been meeting a source, or the obvious, simply walking.
Sure ... he could have had trouble sleeping, thought he had heartburn, needed fresh air, etc.
Granted things are done a little different in California, but what are the odds that:
1. He'd just happen to go for a stroll at around midnight?
2. There would be somebody who not only saw him out walking, but saw him at the exact moment he collapsed?
Still, I'm reluctant to dive into any conspiracy theories until more facts are known.