Apple Watch SE heart rate monitor

During a workout on an exercise bike the apple watch heart rate will jump from approximately 100bpm to approximately 160bpm for a minute or 2 then drop back to approximately 100. This has only just started to happen and does not do it every day. Is there some reason for this as I don't feel any difference to the heart rate whilst exercising. I am a 78 year old male in reasonable condition and feel the heart rate of 100 - 110 is about the right rate and do not feel it should be anywhere near the 160. I ride with my hands on the handle bars with the same tension on the bike and at the same speed each day. Does anyone have any answers please. The watch is only about 2 months old.