There's several sides to this issues that I've seen and even have done myself. I'll talk about a few of them, but there's more than I can cover as well as variations and combination of things that are far too many for even think about.
The other dev could be insecure about their abilities, so they take the easiest things to work on, knowing it's within their capabilities. They do it to try to impress people with their skills as well as getting an ego boost. This isn't always a bad thing, but it can definitely be annoying or even a major problem if they are literally always doing this. Also, if they are literally doing this all the time, they are likely wasting a lot of time looking at the job queue instead of doing their normal work.
Sometimes people just need to take some easy tasks to either prevent or recover from working too hard. This will be something done on a fairly short term basis and will go away as suddenly as it started.
Similarly, there may be some things going on at home that are temporarily increasing their stress, so they need to get some easy stuff on their plate to be able to handle things at home better.
Similar to the insecure dev, and egotist will take all the easy jobs to impress people, but will do it to prevent anyone else from being able to do it, then show the boss how many tasks they were able to perform, as compared to other people. If you have a point system for tasks, they may even be able to rack up a bunch of points this way, while others trudge along with higher point tasks that take considerably longer to do. This type is specifically doing it to further themselves and/or to hinder others.
Sometimes you'll get someone who doesn't have a very good work ethic or simply knows their time at that employer is short. It could be short due to a recent move in jobs that hasn't taken effect yet, the end of a contract coming up, or they just don't care and are willing to get fired to get out of the job.
This is the type of person who sees something really easy and to not waste time of others who are clearly better to be doing harder projects, they take the easy stuff. This allows others to concentrate on getting the real work done.
From what you've described, your co-worker is a bit of a mix between insecure and egotist. They may not be doing this to specifically hurt others, but they seem to be doing it to boost their rank in the eyes of the boss. Which way they lean (as to egotist or insecure) depends on why they are doing it.
Your manager/boss should have a policy on how work is taken, but if there isn't a policy or the policy is vague, it needs to be implemented to prevent this type of "sniping" work. No policy will be able to prevent all of it, but a decent policy should be able to prevent some of it without making people mad. You co-worker may think they are the "helper" type, so continuing to let them know it isn't actually helping will fix this. Knowing why people do things will make it easier to figure out how to prevent them from doing undesirable things.
Some things need to be done in a specific order. If they do things out of order, it'll cause more problems. Make sure people know about these situations. They shouldn't come up very often, but they can.
Not your job
Just to keep things in perspective, you correcting your co-worker is not your job. It's the job of your boss/manager or your team lead. That said, you mentioning it in a non-accusatory fashion can be useful. Offer solutions, instead of whining or suggesting punishments. Suggest a better policy for handing out work. Suggest peer programming for more training. Suggest something that'll show forward movement in policy, whatever it may be, rather than negativity. You should be offering these to your superiors, not your co-worker specifically. You can offer them in a team meeting in a way that all of you can make things better, just don't single out anyone. Doing so will not win you any friends.
There's things that you can do and things you can't do to fix this problem. Even if you do make progress with policy, you aren't going to fix every problem. Anyone who is willing to spend the time will be able to "game the system", unless doing the real/normal is more interesting or less actual work than trying to cheat. And trying to prevent all forms of cheating will likely cause a lot more work for everyone as well as while trying to prevent the cheating.