Montana 700 incorrectly recorded track.

This is not unusual in challenging conditions. Most likely, this is an artifact of multipath reception. GPS signals bounce off of local objects - terrain like bluffs, tall buildings, and so forth. Net result is that the unit "sees" multiple signals from the same satellite. The bounced signals will differ in time from the direct signal, and from one another. This means that the position computed from each signal will differ from positions computed from the others.  The unit does its best to choose the direct signal - but cannot always do so. 

When the unit switches from using one signal to using another, there is a virtually instantaneous shift in computed position. This leads to the type of excursion shown on the map. If you were to check the speed along excursion, you would see ridiculously high speeds as well. The excursion gets added to the track length.

Garmin units are notoriously bad about ignoring such excursions.

Thanks for your comprehensive reply.  That all makes sense.  What is interesting is that this particular area is a low density coastal area with no mountains etc and I normally hike in gullies, ravines, mountains and never seen this at all.  I have been using the Montana’s for years.  You are correct about the speed, the deviation is a yellow line which denotes a fast speed (compared to the rest of the track).  Whilst I like to think I am a reasonably fast walker, I’m not that fast!  Regardless, thanks for taking the time to reply and explaining this so clearly, much appreciated.