[WillKirkby]

It's even more annoying on mobile (I'm using Android).

If I click an Instagram link in a tweet (since Instagram images no longer show up inline on Twitter), it loads the t.co link in my web browser, and then launches the instagram app to show me the photo. If I then press my phone's "back" button, it takes me back to my web browser, and I have to press "back" again to get back to the Twitter app. It also leaves the t.co link in my web browser's list of active tabs, so the next time I open my web browser, it hijacks me and launches me back into Instagram. It would not be difficult for the Twitter app to resolve t.co links before launching anything, mitigating this issue entirely.

[dublinben]

The problem here is that Twitter has hijacked every URL posted on its service with the t.co redirector. Even if you post an already short link, it will be wrapped in a useless t.co, which causes problems like what you've described.