Looking for answers.
The medical standpoint. There is, at present no consensus on the etiology (root cause) of EGPA.
Theories and research only suggest potential causes.
The individual standpoint. Many of those suffering with EGPA can point to a single event which they believe led to it's onset. Possibly the event did
trigger an already underlying potential for the disease.
Supporting research. Understanding the causes will hopefully lead to earlier detection and strategies
for treatment. The charity referenced and other organisations would appreciate your support.
One study indicates that there is a genetic disposition for EGPA with certain gene alleles
(pair of genes that appear at a location on a particular chromosome).
The gene studied is in the HLA region (Human Leukocyte Antigen). The relationship between the HLA region and general autoimmune diseases
has been established for more than fifty years.
It is generally accepted that EGPA is not inherited, however there are isolated cases of more than one family member with EGPA.
Infections and vaccines.
There is little to suggest that infections and vaccines can be a trigger though there are a few case studies that
raise the possibility.
A study in 2003 looked at 75 cases of primary systemic vasculitis (16 had EGPA) compared with a control group.
Factors investigated were social class, occupational and residential history, smoking, pets, allergies, vaccinations,
medications, hepatitis, tuberculosis, and farm exposure.
The EGPA sample was small, so for example none of those 16 were exposed to solvents. Given that the study recognises its own
limitations, for EGPA there was a noticeable increase in incidence with recent silica exposure.
A 2013 study into silica exposure only included the same 16 EGPA subjects, so does not add further clarity.
Certain medications may also be implicated, including macrolide antibiotics and quinidine. However, most notable are the leukotriene
receptor antagonists (LTRA's) montelukast and zafirlukast. 
. More recently Omalizumab a monoclonal antibody that binds
to immunoglobulin E (IgE), has been considered as an EGPA trigger.
These drugs are steroid sparing agents for asthma and there is also a contrary view that they simply unmask EGPA,
due to the delayed or reduced use of steroids. 
It has also been suggested that the presence of both silica and an LTRA could be a strong trigger. 
Whatever the reasons, What really matters is what you do next. We can do something about the future but only fret about the past.