I think they're less standing o's, and more the audience trying to see the action on stage. I believe during the Barker era the show used a maximum of four cameras to film the show. Currently, I think there are 6 cameras on stage, not including the boom cam (? - I'm probably off there, but I seem to recall on Mike Richard's podcast, he stated that was the set up he preferred for filming the show.) Add to that mix, stagehands, producers and a lot of movement; and I think those sitting in the front rows, which are lower than the seats further back, have trouble seeing what's going on, therefore stand up to see, which causes everyone else to stand, so they can see over the heads if the front rows. And they happen to be clapping at the same time.
But then there's also the case, that people standing, jumping and clapping wildly on camera looks good to the home audience, so its most likely encouraged by the stage managers to the nth degree.
If you view episodes per decade, you'll see that in the early 70s, the audience was very demure and polite. As the years moved on, the crowds become more and more lively, especially during the 80s when the show was hugely popular with college students. Audiences kept getting more and more boisterous, so much so, it became as significant a part of the show as bidding $1 or hearing 'a new car' being shouted. If you tried to wrangle the audience's enthusiasm, the show would feel very weird without it, even if it does seem exaggerated at times.