Microwaves do, indeed, "bounce around" within the oven cavity. There
is usually a standing-wave pattern with hot and cold spots, which is the
main reason for the turntable.

Oh, how long do they bounce around tell me. If that were true with
the klystron [sic] continuing to transmit more waves and the old ones
keep bouncing around they would build up and explode the oven. That's
[sic] does not happen does it.

Oh, but it does.

If there's nothing in the cavity to absorb the microwaves, the magnetron
will overheat. The metal walls of the cavity absorb very little radiation,
reflecting most of it.

Now, tell us... What's your real name? (What parents would name their kid
"Claude Hopper"?)

There used to be an old-time-radio company run by a gentleman named Carl
Amari. (Say it out loud.) He was fully aware of the unintentional play on
words.