• worm infestation
• incorrect feeding (for example: silage feeding, far too little hay, moldy hay...)
• stress
• diseases of the nervous system
• bacterial diseases (e.g. Lyme disease...)
• viral diseases
• foreign bodies in the feed
• toxic substances (e.g. poisonous pasture plants, wood preservatives, formaldehyde in bedding, heavily over-fertilized pastures, fungal spores in hay)
• after medication (e.g. antibiotics, cortisone)
• excessive pesticides (plant protection products) or nitrates in the feed
• increased synthetic mineral feed (for example over 200 grams daily)
  

What else you should consider:

• no fruit and vegetables
• If possible, do not feed a second cut of hay, only the first cut. The second cut can cause severe diarrhea. The hay should be left to rest for about 6-8 weeks after the hay harvest before it can be fed.
• The horse should always have roughage freely available
• during this time, do not give mineral feed or salt licks
• In case of intestinal disorders or intestinal diseases, one should avoid grains (except oats in small quantities)
• do not feed silage