aaslyfox is right, you need to protect the topic. This should keep anyone who isn't an op from changing the topic

on 1:text:@nt*:#: {
  if ( $nick isop $chan) {
    set %nt $rand(1,6)
    if (%nt == 1) { .topic $chan 13??*~`11~*??|??*~`8~*??|??*~ 7-=?=-9 $2- 7-=?=-8 ~*??|??*~`11~*??|??*~`13~*?? 11 }
    if (%nt == 2) { .topic $chan 4,4.4,1*8,8.8,1*13,13.13,1*9,9.9,1*4,4.0,12~ $2- !4,1.8,1*13,13.13,1*9,9.9,1*4,4.0,12 }
    if (%nt == 3) { .topic $chan 13_?,.??=???`? ?`?????=??.,?. $2- _?,.??=???`? ?`?????=??.,?. }
    if (%nt == 4) { .topic $chan 11-=<{([ 12 $+ $2- 11])}>=- }
    if (%nt == 5) { .topic $chan 12-?0,12?2,12`?12,2?1,2`?2,1?0,1 $2- 2,1`?1,2?12,2`?2,12?0,12`?12,0?- 11 }
    if (%nt == 6) { .topic $chan 4,0?0,4?5,4?4,5?1,5?5,1?4,1 $2- 4,0?0,4?5,4?4,5?1,5?5,1?4, 11 }
  }
}