There a many different things that could be causing this problem, including things like liquid damage. However, as you have an iPhone 5, the most likely scenario is that your battery is failing. Failing batteries will appear to be normal when charged (they will look like full batteries), but will quickly drain and power off. You can take your device to an Apple store, and they can test the battery for you. iOS batteries typically last around 2-3 years.

I spoke with Samsung Customer Service support about this same issue. The person who helped me said there is a spring underneath the trackpad that gets dirty and causes it to stick. According to her, you just open the unit, remove the trackpad, and clean that spring with denatured alcohol. I haven't tried it yet, though. Will be trying that repair shortly.