Does pedaling while standing wear down the pedal? I'm a new user. I searched for this question but couldn't find it.

While I was riding today, about 10 miles in my pedal spontaneously popped off. I was later able to repair it with a wrench, but I'm concerned that pedaling while standing excessively could have caused this. I'm very careful not to place a disproportionate amount of weight on one side or the other. Only one pedal fell off, the right one.

Is this something I should worry about? My bike is less than a year old.
No, pedaling while standing will not cause a properly installed pedal to come off. However, if a pedal comes off, it not particularly desirable to be standing.

As far as weight distribution, putting all your weight on one pedal for extended periods will not hurt you bike or cause things to go wrong with the bike.