It is not the crystal that gets damaged, it is the output power amplifier. Your transmitter is designed to radiate the power as RF. When the antenna is down, the SWR (standing wave ratio) goes way up and the power that would normally be dissipated as RF is now radiated as heat in the output stage. You probably notice the back of your transmitter getting quite warm if you run for very long with the antenna down. You should always run with antenna FULLY extended except for the short time required for a range check. Note that damage by excessive heat can be cumulative; i.e. won't fail all at once, just dies a slow death. The mean time between failures will approximately double for each 10 degrees C that the output stage rises.