Why was this horse being sooooooooooooooooo mean and stupid? I did everything right!?

Ok so there is this like 25 year old horse at my barn I board my horse at. I rode him today because his owner was away. I used my tack because his tack was brown and brown doesn't look good with me so I used my black saddle. I was jumping him around 4 feet because that's wat me and my horse do after a while he got tired so i did a hack on him and went through a creek to cool off then i went to the big open pasture and started to gallop him he looked tired but i kept him going then he suddenly bucked me off! Then he lay ed down and took a nap! Why did he do this? My horse can does this easy, I gave him breaks! Why was he SO mean, im hurting now!