I think we need clarification if 'primary' in the context of the speedway exclusion is actually used in the context of 'first' or more in its context of 'main' or 'chief importance' (or whether it's indeed left deliberately ambiguous)
primary
ˈprʌɪm(ə)ri/
adjective
1.
of chief importance; principal.
"the government's primary aim is to see significant reductions in unemployment"
synonyms: main, chief, key, prime, central, principal, foremost, first, most important, predominant, paramount, overriding, major, ruling, dominant, master, supreme, cardinal, pre-eminent, ultimate; informalnumber-one
"the police believe that crime detection is their primary role"
2.
earliest in time or order.
"the primary stage of their political education"
synonyms: original, earliest, initial, beginning, first; More
Because primary does not always mean first. I can see both sides - The first incident was the engine failure of course, but I can see the argument it doesn't necessarily mean it was the incident of chief importance which actually caused the stoppage.
Perhaps in this case it is slightly more clear, but certainly in other situations the first (or primary time wise) incident isn't always the incident of chief importance which actually stops the race or results in the exclusion.
Can see both sides, interesting debate.