The usual way a barrier fails is that it falls over or is broken or is taken to pieces. Thus the things which it ought to keep apart can converge. In car racing I think it might be a wooden fence which fell over or a metal obstacle which shifted or a pile of old tyres which became instable, that sort of thing. In any case without proper barriers between the racing area and the spectators it would be too dangerous to carry on the race before the barriers have beenb repaired or replaced.