Replacing 2 dimmer switches, breaker keeps tripping?
Congratulations, you've created a short-circuit.  You've connected the ungrounded (hot) conductor directly to ground through the switch. 

the reason the second switch caused the breaker to trip, is because it was toggled in the "correct" orientation.  If the other switch had been toggled, it to would have tripped the breaker.

What you need is a single pole switch, not a 3-way.  Then you'll connect the green wire to the green grounding screw, and the hot wires to the other brass colored screws.