Whenever I've done this in the past it was because of a desire to do a good job.

Whenever I find myself working for someone where the majority of what I do is because I fear retribution I've also found that I've generally grown unhappy with the situation and should move on.

Work isn't 100% fun, and it probably shouldn't be. There are going to be tedious things that you HAVE to do because it is your job. However, if you're doing everything because you HAVE to, not because you want to do a good job, there is a problem somewhere and you should examine it.

I think it's way more than that. There's a number of ways to motivate people, such as involving them in the team mission and empowering them, compensation, and recognition. Motivated people will do what it takes, including thinking about work off hours.

I'm on holiday visiting family, and travelling back home at the moment. I need something to do for the 9 hours or so in travelling for, and work emails are one of the things I use to kill the time.

9/10 times for me it's because I'm filtering them out. It's not a good feeling to go back to work after a few days being out to a full inbox.

The other 1/10 of the time is because I probably pushed something out last-minute before I left due to some deadline, and I want to make sure my coworkers have their questions answered. I don't necessarily look at it as a fear of being fired, more so as someone who has been on the opposite end of that scenario and it's frustrating. If spending 30 minutes answering an email from a coworker on my vacation prevents a day's worth of frustration on their end, yeah, I'll take one for the team.