Look in the terminfo database for your terminal for the key that sends this escape sequence. The
infocmp command dumps the terminfo entry for the current terminal.
$ infocmp | grep -oE ' k[[:alpha:]]+=\\E\[Z,'
The terminfo man page explains what
cbt is the abbreviation of. (It also gives an example which corresponds to most terminals out there.)
$ man 5 terminfo | grep -w kcbt
key_btab kcbt kB back-tab key
kbs=^H, kcbt=\E[Z, kcub1=\E[D, kcud1=\E[B,
So you have it:
\e[Z is backtab, i.e. Shift+Tab (on most terminals).