I have no idea what the causes were specifically, but Curiosity was over two years late in launching (planned Sept 2009, actual Nov 2011), per:

https://en.wikipedia.org/wiki/Timeline_of_Mars_Science_Labor...

When it comes to Mars, though, even if you’re only a month late it will normally result in a 26-month delay due to how often (or rarely) launch windows open for Earth–Mars Hohmann transfers, and this is exactly what happened with Curiosity.

Here's more on why: https://www.thespacereview.com/article/1319/1

TLDR: It was not the EDL [edited to add: entry descent and landing] system, it was the fabrication and integration (putting together) of certain actuators that are used in low temperatures at Mars. People at a contractor, and at JPL, were putting in double shifts and they came within a very close margin of getting it together.

Additionally, there were flight software/avionics issues.

But if you're set to miss the launch window by even a week, you have to wait for the next one 26 months later.