Why did my circuit blow after replacing a switch?
There are only a few possible causes:


The newish switch was not wired correctly.  I think this is the most probable cause:  anything recently changed and not working should be reviewed.  Maybe there is a stray strand of wire between the switched terminal and ground, or some wiring fault in the lights.  If any of these are the problem, turning on the switch will trip the breaker even if the dehumidifier and other devices are off.  That is, turning on only this switch is enough to blow the breaker.
The breaker has derated itself&mdash;that is, it is marginally defective.  This happens sometimes, especially with heavily loaded circuits.   Breakers get hot, or if they are tripped many times they can experience something similar to heat fatigue.
The load has increased:  more bulbs or more wattage, or the humidifier is using more power.  Many electrical systems do not provide steady voltage.  If it has increased recently, from say 114 to 122 volts, that might be just enough to increase the power to where the breaker should trip.


If it were me, I would


Shut off the circuit  
Pull the "new" switch and examine it for physical problems.
Use a DMM to check for continuity between the black wire and ground: there should be none&mdash;that is, the ohms should be infinite.
Disconnect the white wires (should be a wire nut in the switchbox) and check for continuity between white going to the lights and ground: should be none.  The ohms between the white wire going to the breaker panel and ground should be close to 0.0.
Check the resistance of the light circuit with the DMM between black and white.  For 120 watts incandescent @ 120V, should be 8&ndash;10 ohms (cold tungsten has its resistance of about one fifteenth its hot temperature).


If the problem had not been found after that, I would reconnect the white wires (undo step 4), turn on the breaker, and put a clamp-on A.C. ammeter (actually an analog meter would be better for this test) to measure the current on a black wire, and turn the switch on.  It can either spike strongly or not at all.  Strongly is bad and means there is a fault in the wiring or fixtures.  Not at all means more tracing of wire, perhaps from the circuit breaker panel is needed.