Why has Hudson been renamed to Jenkins?
The Jenkins website has a blog post on this issue:

But one issue, which we feel is the most significant issue of all, one for which we now believe no resolution is possible: the rights to the name Hudson.
Oracle has told us that they have trademark applications filed in both the EU and US for Hudson, based on Hudson's creation by Kohsuke while working at Sun. The problem is that this trademark ownership gives Oracle the ability to revoke the Hudson project's right to call itself Hudson at any time, and while Oracle has made an attempt to offer some guarantees (most notably, that binary releases of Hudson, once they've been released with the name Hudson attached, will always retain the right to the name), they are not offering any binding guarantee that the Hudson project will be able to retain its use of the name in perpetuity.
Therefore, to continue using the name Hudson means ceding some of the project's independence to Oracle - if the project and its governance board opted to go in a direction Oracle disapproved of, Oracle would be able to take away the naming rights. Or, in a less dramatic scenario, Oracle could insist on certain changes to the code, infrastructure decisions, process, etc, regardless of opposition from the Hudson development community, in order to retain the rights to the name.

The Jenkins team were concerned that using the name 'Hudson' would give Oracle far too much control over the project, because there was a constant threat that if the project didn't do what Oracle wanted, they'd simply yank the rights to the trademark.
In order to retain independence, the developers of Hudson voted to rename the project Jenkins. However, here's where it gets confusing. Oracle chose to continue developing Hudson, essentially making Jenkins a fork because the code started to diverge. There is some dispute about whether this is really a fork or a rename, although it's more an argument over semantics than constructive discussion.
As of now:

Jenkins has become the predominant version, and interest in Hudson has collapsed

Hudson is now managed by the Eclipse Foundation, after Oracle gave up development and donated the code to Eclipse.