crack in newly sanded and polyurethained floors

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hi there
I had my floors sanded and polyed last wed and thursday.I noticed a small crack today(5 days later) in one of the pieces of wood.not huge,almost like a small hairline fracture.why would this happen?could this be from walking on it or putting stuff on the wood?

I know it definetly was not there when the guys finished the job.could that happen after the fact?

thanks
sandy