The important piece of org-mode configuration is:

    org-minimal.el

    (setq org-todo-keywords '(
      (sequence "TODO(t)" "NEXT(n!)" "|" "DONE(d)")))

So I expect the following note to appear in LOGBOOK once I change the state
from TODO to NEXT:

    :LOGBOOK:
    - State "NEXT"       from "TODO"       [2015-02-04 Wed 22:11]
    :END:

It works perfectly fine if I change a state of a single task.
But if select multiple TODO tasks in Agenda view and change all of them to
NEXT in bulk, only the last task will have such "state change" record.

I expect org-mode to create such "state change" note in LOGBOOK drawer for
every task I'm changing.

Now I'll describe my steps in a bit more detailed way, and afterwards you
will find the system information I use.

1. emacs -Q -l ~/org-minimal.el ~/test.org
2. Create 3 tasks:
    * TODO task1
    * TODO task2
    * TODO task3
3. M-x -> org-agenda-file-to-front
3. M-x -> org-agenda -> t
4. Mark those 3 tasks
5. M-x -> org-agenda-bulk-action -> t -> NEXT

The ACTUAL contents of ~/test.org is:

    * NEXT task1
    * NEXT task2
    * NEXT task3
    :LOGBOOK:
    - State "NEXT"       from "TODO"       [2015-02-04 Wed 22:11]
    :END:

The EXPECTED contents of ~/test.org is:

    * NEXT task1
    :LOGBOOK:
    - State "NEXT"       from "TODO"       [2015-02-04 Wed 22:11]
    :END:
    * NEXT task2
    :LOGBOOK:
    - State "NEXT"       from "TODO"       [2015-02-04 Wed 22:11]
    :END:
    * NEXT task3
    :LOGBOOK:
    - State "NEXT"       from "TODO"       [2015-02-04 Wed 22:11]
    :END:

Emacs: GNU Emacs 25.0.50.1 (x86_64-pc-linux-gnu, GTK+ Version 3.12.2) of
2015-01-09 on lgw01-32
Package: Org-mode version 8.3beta (release_8.3beta-794-g55c070 @
/home/z0rch/.emacs.d/el-get/org-mode/lisp/)