Check the crotch or the stems where they come together on the trunk. Are there any small holes? There are worms and beatles that bore into the stem and they will kill one branch at a time. Probably not ants unless they are rasta carpanta
Also, it looks like some of your plants have outgrown the space. They seem to be hitting the roof of your structure. How hot does it get in there? Is the whole area closed off? Looks like steel fencing and some type of transparent tarp.