Elvis Pfützenreuter - home page
It happens because the current leaks to other columns, running "backwards" through switches. For example, when A1, A2 and B1 are pressed, the "ghost" B2 key is also registered.
The lack of diodes is not a showstopper for "office" keyboards, it is more of a problem for games where 3 or more keys may be pressed at the same time.