Toddler peeing on carpet

[sarahe11887]

My son is 3.5 and has been successfully going to the potty for about 4 months.  He sometimes has a poop accident in his pants, but has been great until recently about peeing in the toilet.  He is healthy and smart, but quite independent.  For instance, he always just goes to the restroom on his own, without telling me he needs to go.  Two weeks ago, he stood on our bed and peed onto the rug below.  He also then peed on the carpet in his room and in the living room.  He still pees in the toilet most times during the day, and he wears a "night-time" pull-up for bedtime.  When he first started this, I talked to him sternly.  Then I had him start patting the area dry with a towel and threatening to take away toys for repeated incidences which I followed up on.  He is continuing this behavior.  He is also not wanting to put back on his underwear after he pees on the carpet.  Today, he peed twice in his underwear without attempting to take them off.  He talks some, but does not give any indication why he is doing this remaining silent and uninterested as I and his dad try to discuss it with him.  Any suggestions are much appreciated.