iPhone 6. Some recorded video clips go into slow-mo on playback.

My wife shot about 20 movie clips at a carnival.

On transferring them via Airdrop and playing them back about a third went into slo-mo about 8 seconds in and returned to normal a similar time before the end. On playback (iMac) of the ones affected there's a dotted line under the normal 'playback progress' line with markers on it indicating where the slo-mo starts and ends. These markers can be moved about and if they are brought together at the end of the dotted line, then the slo-mo disappears. But, those settings are not saved by clicking on 'Save Alterations' and if one goes back to it in playback then the slo-mo is still there. It doesn't matter how long the shots lasted, the indicators for the slo-mo are all the same. A bit odd..? Can anyone advise, I am a bit of a newbie to the iMac.. The only possible glitch was that the phone memory was running low, but can't see that would make this happen..