AIX

those seem to be orphan process, it is better to wait for a reboot

I don't use AIX but in general an "exiting" process has begun to run the exit system call. Killing it won't help, it is already trying to exit. Of of the things that exit() does is to close all open files and my guess is that you are hanging on that. I don't know about AIX, but on Solaris and Linux and HP-UX, we can use a program called lsof to list the open files of a process. I would try to find out which files are still opened. Opened files on a non-responsive NFS server are indeed a very good bet. Once I was sitting at a system console and my knee hit the power button on a disk drive. This caused some exiting processes and a lot of other trouble as well. I powered the drive back on and it recovered.

This may or may not be an orphan. Orphans are quickly adopted by init and there is nothing wrong with being an orphan. Non-orphaned processes risk being zombies if the parent does not wait for them. But in any case, a process must complete the exit syetem call to finish dying. One of the last tasks of exit() is to notify the parent of the death of a child. If the the parent is swapped out, it may need to be swapped back in to receive the notification. If there is no room to swap it in, the kernel must wait until room is available and exit() could hang for this reason. But there would be more indications of trouble if this was the case.

If you're running system accounting, the exit call needs to write a record in the accounting file to record the process exit. I have often wondered what would happen if the file system was full and the record could not be written. But this seems to have the potential for trouble as well. And these are the only things I can think of that might cause the problem.