Someone at Linode did it. Ran a script to see how many bitcoin files there were on all the machines (they probably do these types of queries for anti-virus/whatever anyways) and took a customer support password to log in and get the coins. If he did it right he still might be working there, as it is easy to get credentials from friends/coworkers (even though it should be really really hard).
I'll happily eat my words if that turns out to be what happened, but it is definitely not the simplest answer.
It is technically possible for an employee to do it because it seems (from the linked pastebin above) that is how it was compromised, an elevated account for linode manage was compromised.
As for an employee being the one that did it, that is probably the least likely cause.
I wonder how anyone can trust their linode systems after an admin account being compromised.
It would likely ruin their business to re-install everything, but that is the only way to know root kits have not been installed.
We can only speculate at this point.
The simplest answer is probably that one of the staff was subject to a targeted hack and a 3rd party gained external access to the CSR tools.
Possibly for an extended period of time. <-- This is the concerning part.
It's relatively unlikely an internal staff member would do something this dumb (but, not impossible. we've had this happen _here_ where I work, with credit card numbers, but obviously the person responsible was caught almost immediately).