Basically he had nothing better to do one day, and instead of carving a joint of meat and enjoy a good lunch with his family, he decided to cut a human being instead. He asked a short man living next door to him to volunteer for the experiment by paying him a few pence.
"Will it hurt?" asked the short man.
"No, it is a completely armless procedure!" replied the good doctor.
For good measure he invited a few other doctor friends mainly for amusement, but when they discovered that Rembrandt would also be there painting the whole event, several of the doctors paid good money to be included in the painting. Fortunately for Renbrandt they all looked the same with pointed beards, so he agreed for all of them to be there since he could copy paste their faces over and again. To distinguish Doctor Tulp though, he asked him to wear a hat.
Cutting people up to learn anatomy was a yearly event in Amsterdam, in Holland and this one is dated 16 Januray 1632. Usually an audience attended the event to make notes and learn about different parts of the body. Real blood was used because tomato ketchup had not yet been invented at the time.
As already mentioned, the doctor in question was called Tulp. Originally, his name was Tulip but in one of these operations he accidentally cut his I and was hence known as Tulp.
Since that sad incident he unfortunately was no longer able to tip toe through the tulips, or through anything else for that matter.