Jordan and Syria massacred thousands of Palestinians
Of course the assassination of the Jordanian King and attempts to overthrow the governments of both countries had something to do with that
Palestinians do not play nice when they are guests of other countries
The PLO tried to overthrow King Hussein in 1970; they were heavily armed and had a mutual non-aggression agreement with Jordan that Jordan was living up to and the PLO was not. Hussein was down by some Dead Sea village or facility, carrying out some kind of inspection tour, when the attempted coup started. He got out alive, but had to pull his own sidearm, and he and his group fought their way out. There were two such attempts on his life. In September he told his army to attack -- hence the name "Black September" btw -- and it started to kick the ever-lovin' crap out of the PLO. Terrorists were fleeing for their lives, dropping their weapons and surrendering to Israeli border guards. Meanwhile Assad, who was in on the planning, had tanks and other forces rolling toward the Jordanian frontier in support of one of the towns the PLO had taken over (and sporting phony PLO paraphernalia), but Israel's air force started buzzing them and Jordan's army attacked on the ground, and got them turned around and running back to their dictator. The expulsion of the PLO saw them leave for Lebanon, still heavily armed, and they then destabilized Lebanon to started the civil war there, which Assad used as a pretext to invade and occupy the country.