Grabbing and holding the Redmi 1S can be tricky and to a point, annoying due to it's sensitive volume and power buttons. And especially when taking in and out of a pocket or bag. I have experienced multiple phone reboots and power off from accidental touches. Some users also accidently went into 'Fastboot' mode. There are two solutions to this problem.
Protective cover
First solution, buy a protective/flip cover that will prevent accidental button touches. Best to just head over to a mobile phone accessories shop to try on a cover before you buy. The Xiaomi soft silicone case(TPU) does not work.
Button settings
Second solution is to disable your power button when lockscreen is activated and enable 'Prevent Pocket Dials' option. These settings are available in the Redmi Note as well. Here's how.
First setting : Goto > Settings
Under 'Device' section, goto > Buttons
Enable 'Disable Power button menu'. The switch will turn orange when the option is enabled. Let's now proceed to the next setting.
Second setting : Goto > Settings > Display
Enable 'Prevent Pocket Dials' option. The switch will turn orange when activated.
Hopefully that will solve the mysterious reboots and fastboot mode problem for you.