Gotti thought Paul was going to whack him and/or his crew for the violation of the no dealing rule.
Or even worse tPaul was going whack Gotti'S ego by busting he and his crew down and separating them.

Castallano was on trial with the Demeo crew and was indicted or about to get indicted for the commission case.

Whether Castallano showed to Neil's wake or not I do not think it mattered.

No Neil should have gotten promoted to boss when Don Carlo passed or that's what the majority of the family thought.
Castallano got the promotion because he was Gambino's brother in law.

The family was divided into two factions 1) The white Collar (Catalleno) 2) The Blue Collar (Dellicroce)

The family was on brink of war for a long time it it weren't for Neil's respect for the rules and the fact he was very old school there would have been a war.
That is part of the reason some of the old timers backed Gotti after the coup.

Chin and Gas plotted to whack Gotti and that was it.
Gas and some one he had hired thought Decicco was Gotti that day and that's why they blew him up.

There was no formal contract put on Gotti or his crew. There was a plot behind his back.
This was done back door and behind the scenes to make it look like Sicilians from Italy did the hit.
USA bombs were outlawed by the commission becaus it brings negative attention.