Dimmer burned and the breaker tripping on an existing LED circuit
The LED &quot;circuits&quot; that landed on the same breaker may have been interconnected at a switch box, who knows why, but maybe somebody else didn't know what they were doing.  At any rate, when you re-arranged the breakers, you probably moved one to the opposite phase of a 240 v circuit, resulting in supplying 240 volts to a 120 volt device. They don't like that!
BTW, why change it? Power consumption by LEDS is so very low that you can practically run an entire house's lighting on one circuit. I prefer at least 2, but I don't think you had a problem initially. What was the problem you were trying to fix?