How does Twitter determine its trends?

Would you think they should be #iwatch or #makemoney or something that you know is trending or at least it should? Instead, it's something as silly as #MusicMonday. Their trends, overall, make no sense to me.

 

Twitter doesn't make the trends, the user does.
The trends aren't always hashtags either, they can just be words used in tweets. Twitter obviously has an algorithm to determine what people are most talking about and can advise what's popular in your geographic area based on this.
Usually you'll have explicit hashtags that may be stated by a TV programme for example. It's now common place to see the hashtag presented on TV before the program. Alternatively people will just guess what the hashtag would be and it goes viral and those using the least common one switch to the one that's being used the most to spread their tweet and get it seen by more people on twitter.

 

So Music and Monday can become MusicMonday. Ok, I get it.

 

As competitors are training for the competition, they tweet with some help from paid staff. They put pound signs in front of catchy phrases to create hashtags, like #TeamNakia. "They had them really operating fast. ... They had hashtags going and they were asking questions and doing polls and really engaging stuff...If you start trending on Twitter, people on Twitter will turn that show on.