I also have the same issue. I destroyed my VPS and recreated it with a few different versions of CentOS: 5 32bit, 5 64 bit, 6 32 bit, 6 64bit.
This only happens for CentOS 6 (both 32bit and 64bit).
An Apache process is spawned and will sit there ramping up in both CPU and memory usage until the plugin is installed (or updated). Once the installed, the Apache process doesn't die off. It no longer has a high CPU, but it keeps a lock on the memory that it used when it was running.
For instance, I just installed tinymce, it finished fine, but there is still an Apache process that is using up 41.8% of the memory. I have to manually kill the process or else I run the risk of running out of memory.
I believe this is a WordPress issue...