V + まくる is slangy and similar to V + すぎる [V a lot or too much]. Sounds the next higher lever of V + すぎる これはきっと抱えまくったストレスと... ~ 抱えすぎたストレスと This certainly ...[sth] stress I (or you or sb) has been holding too much. これはきっと抱えまくったストレスによる。 This certainly comes from stress I have been holding too much. This certainly comes from too much stress I have been holding. Hope this helps.

In this case, I think "～まくった" means "～ more and more" or "～ higher and higher" or ”too much”. So, Ithink... He guess the reason somethig is person had too much stress and he stood it longer.