What Killed the Kalamansi?

I wonder what killed my mature Kalamansi tree (Calamondin)?  It was at least 5 years old and it had a few fruits last April, so I was lulled into thinking that it didn't need my attention.  It's true that I had not watered nor fertilized the plants in my yard this year, and the summer heat has arrived, but that didn't stop the rest of my garden from blooming.  Why not the Kalamansi, I wonder?

Everything is green but the Kalamansi tree.

 

The dried-up fruits on my dead Kalamansi tree.  Sad.

 

Orange and purple Trumpet Vines (Campsis Radicans) near the Kalamansi

 

 Hydrangea near the foot of the Kalamansi

 My neighbor's aggressive -- yet beautiful -- Pink Bower Vine (Pandorae Jasmonoides) has climbed over to my side of the fence.

 My magenta Bougainvillea that didn't look very promising last year has suddenly blossomed and has reached over to my other neighbor's trees.

It's looking good for my White Bower Vine (Pandorae Jasmonoides) too.