Although I am probably going to get a lot of flak for this answer, I believe this explanation fits the criteria of your question-specifically going to “complications due to being too close to an explosion”. This type of reaction is not restricted to soldiers on the battlefield. Please read through completely before rejecting it out-of-hand.
Concussion is also known as mild brain injury, mild traumatic brain injury (MTBI), mild head injury and minor head trauma.
Concussion causes temporary loss of brain function leading to cognitive, physical and emotional symptoms, such as confusion, vomiting, headache, nausea, depression, disturbed sleep, moodiness, and amnesia.
Among the causes of concussion we find...
Explosions - concussions caused by explosions (bombs, grenades and mortar shells, for example) are common among veterans who have served in Afghanistan and Iraq.
In December 2015, the Department of Defense reported that there have been 339,462 medical diagnoses of concussion among members of the US armed forces from 2000-2015.
Repetitive blast exposure tied to brain changes in combat vets
Brain injury experts find that combat veterans exposed to repeated mild explosions show chronic changes in neuron activity in certain brain regions - and the more blasts they are exposed to, the more of the lasting changes they show.
Long-term after-effects of Grade 3 concussions may include irritability, apathy, sudden rages, depression, confusion, memory loss and (it is thought) Parkinsonian symptoms in some cases. This has been referred to PostConcussive Syndrome (PCS).
Not everyone who has been through an explosion develops PTSD; but for those who do, it is possible there is a physical reason for the disorder.