There's nothing explicitly stopping people trademarking "Dev Mode" but there's all sorts of things around which Goods and Services you're using - which there must be related to your business and a thing called "Proof of use" which is what is sounds like - which means you actually can't just trademark all the words in the dictionary, you have to be actively using it on relevant things.
Saw some comments on twitter saying "oh but xyz used that term in 1998" - that doesn't get taken into account since at that point it's not a trademark. You don't have to be first, just the first to trademark it + it has to be relevant to your business and you have to be using it ;)
The clashes are down to an assessment of aural, visual and conceptual similarity (some fun NLP in there!) if multiple corps use the same terms, or similar terms.
Oddly terms like "Apple" get extra protection for being well known names associate with clear brands.