As a state vet for many years I have had some wild times with horses that fell, were unhurt and ran away (with out the bridle because it got pulled off in the fall) or had heat strokes and they do get wild and hyper (ie the nervous system is over heated causing hyper excitabiltiy)
but never had one get into the in field lake.
Pretty smart horse to go cool him self off in the lake, but also lucky it was a shallow lake.
An old outrider I knew told me a story back many, many years ago a filly trained by Horatio Luro got loose in the Atlantic City Race Course infield and was chased until she jumped in the lake and could not get caught because she was in such a panic. She would not come near any people tring to help her and she drown.
No matter how long one works with horses, one never really ever "sees it all" because horse will always suprise you!!