Why do I suddenly start losing lots of Trophies?

The more Trophies you have for your level, the bigger target you are.

Most people will choose their opponents (unless they are looking for a specific resource) depending on who gives them the most Trophies with the easiest battle. So the more Trophies you get, the more likely people will attack you, and leave someone who only gives them 3 alone.

Since there is an average Trophy amount for each Ascension Level, there is also point where you will be worth 25 Trophies to most players at your Ascension Level. If you reach this point, you can expect to be attacked a lot.