Killing an Arduino and I2C device with a relay?
As you switch off the relay, there suddenly is a very high \$\left\lvert\frac{d\,i()}{dt}\right\rvert\$. Now, the coil of that relay is an inductive device, and inductive devices have a voltage-current relationship of

$$v(t) = L\frac{d\,i()}{dt}\text.$$

Hence, your transistor see a potentially huge (positive) voltage spike, and might fail in the process, taking the IC with it.

The usual way of dealing with that is having a relatively beefy diode antiparallel to the coil.