A friend of mine told me he had seen a documentary about the free diver Herbert Nitsch. He said that Herbert has suffered a decompression sickness after trying to beat his own free dive world record. I wasn’t believing him so I started to use a search engine and found out he was right.
DCS happens if you have plenty of nitrogen in your tissues and you ascent too fast. The nitrogen leaves the tissues too fast, which generates huge bubbles that harm your body.
In scuba diving you are staying down for a longer time and you are breathing constantly air/nitrox and therefore plenty of nitrogen with a high pressure. Every PADI open water scuba diver will learn to never exceed a ascent rate of 18m/min to get ride of the nitrogen slowly. Furthermore you should always perform a safety-stop and should stay at the surface for quite a bit before going down again.
Herbert had just taken one packed breath for the whole dive, but went down to 253m and had a way higher ascent rate. At the end he had planned a decompression stop at 10m, but because he was unconscious a part on the way up the safety diver lead him straight to the surface. He got multiple stokes, but survived it.
You can also suffer DCS if you free dive to often with too little surface time. Your tissues will constantly build up nitrogen and just release parts of it every time you hit the surface. If you overdo it to much you will feel the symptoms either already inbetween the dives or afterwards.