The implementation of the += method uses remove(key: A, elems: List[(A, B)]): List[(A, B)], which is not tail recursive.

A quick way to reproduce it in the REPL:

scala> val m = collection.mutable.ListMapString, Int
m: scala.collection.mutable.ListMap[String,Int] = Map()

scala> var i = 0
i: Int = 0

scala> while(i < 80000) { m = ("foo" i) -> i; i = i+1}
java.lang.StackOverflowError
at scala.collection.mutable.ListMap.remove(ListMap.scala:56)
at scala.collection.mutable.ListMap.remove(ListMap.scala:56)
at scala.collection.mutable.ListMap.remove(ListMap.scala:56)
...