If you summon an entity with a passenger entity with a passenger boat like so:

/summon Zombie ~ ~ ~ {Passengers:[{id:"Zombie",Passengers:[{id:"Boat"}]}]}

Any server will crash if the first entity is able to sit in a boat. So for example if you use this command:

/summon Squid ~ ~ ~ {Passengers:[{id:"Zombie",Passengers:[{id:"Boat"}]}]}

It will not crash.

The only possible explanation I can think of is that the boat is trying to pull the lower entity on it, but because it's technically riding it, it can't. It's constantly trying to do this over and over again which causes the crash. The boat wont try to pull the entity it's riding on in however.

Code analysis by marcono1234 can be found in this comment.