Tasers often seem to be used early on in a confrontation in the US - the use of force continuum for many states allows police to use Tasers if the suspect "pulls away or tenses during handcuffing", but the majority (75%) also allow it to be deployed against fleeing suspects - unsure if Slager's PD allows this (reference here if interested:
https://www.ncjrs.gov/pdffiles1/nij/232215.pdf) . Therefore by running, Scott is making himself far more likely to get zapped, and it's pretty obvious from the dashcam vid that there was no threat to Scott of a Taser or any other force being used against him when
he chose to run.
So it's possible Slager used the Taser to attempt to stop Scott from fleeing once he caught up, but it appears there was no obvious duress or other stimulus that initially caused Scott to run from Slager.