I've used the same recipe for years: soaked ground fava with enough chickpea flour to bind, sesame seeds, salt spice, baking soda. Using thawed frozen portions of the mix, sometimes I get little cauliflowers instead of balls. Next try, from the same batch, all perfectly fine. They never disintegrate; just crack open after a minute in enough oil to cover. I believe my uncrowded oil temp is stable at 170ºC. Is this a temp issue between cold raw falafel and hot oil? Maybe baking soda not distributed well, expanding too much? I have thrown raw balls not entirely free of ice crystals into hot oil without it happening. It seems so random, what could be the issue? 
Fundamentally, the falafels crack open because the outside wants to shrink (due to dehydration) and the inside wants to expand (due to evaporation, gas expansion, and the action of the leavener). The variation you see -- some managing to stay together, others cracking -- is likely due to things subtly affecting the cohesion, such as the distribution of sesame seeds. So I wouldn't suggest trying to determine why the effect is intermittent... it's balanced on a knife edge, and sometimes it falls off the wrong side. Using more chickpea flour and less fava beans might help. Grinding the fava beans more finely might help. Adding some wheat flour might help. Basically, anything you can do to promote cohesion. Additionally, reducing the baking soda might help, but it's not the first thing I'd try. In a comment, Joe makes a really good point that the shape determines how much strain the expansion/contraction causes. A sphere is already at minimal surface area, so expansion has to involve cracking. But a smooshed sphere (or disc) can bow outward instead, flexing instead of rupturing. 