Experiments are presented that show details of spontaneous formation of the crossing wave pattern in the thin layer Belousov-Zhabotinsky reaction-diffusion system, together with a model that qualitatively reproduces the phenomenon. The model shows that small variations of the layer thickness can result in variations of the excitability profile. The crossing wave pattern emerges when an initial periodic bottom wave train excites the top part of the layer along a line where a small sharp increase in the thickness of the layer takes place.