I think incels are a group of sufficient size that you can just state relatively is that all of the above are likely to be true to some portion of them whether that be extremely negative experiences with women such as abuse from a parental figure, they mentally don’t connect well with the women they meet and thus are unable to form meaningful relationships or they just fell into it and are on the edge and not in the deep black pill stuff but identify with the word.

…europa.eu/…/ran_cn_incel_phenomenon_20210803_en.… presents three main ideas (on page 4) being

history of abuse/mental trauma, social skills deficit and/or lack of awareness/distorted boundaries.

psychologytoday.com/…/new-research-on-why-incels-… Whereas this one (looking for the extremist and violent incel subcategory of incels) basically hits upon every possible reason from abuse to indoctrination to just active toxic masculine.