Standing wave ratio is a measure of how efficiently power from the
radio is being coupled into the antenna. A high standing wave ratio
is a result of impedence mismatch between any or all of the radio,
the coax and/or the antenna. Any power that is reflected back (not
coupled into the antenna) will travel back up the coax to the radio,
and, since it is out of phase with the forward power, will produce
a standing wave on the line, even on the outside of the coax.

Needless to say this is unsatisfactory and these standing waves can
radiate into other nearby equipment. Wet coax and bad connections
are the usual cause especially in a setup that was previously working.
One culprit is the coax balun... and/or the antenna connections
themselves. If your coax and connections are old you are better off
just replacing them all.