to suddenly understand (esp. on basis of experience or memory); to come to mind; to recall (in a flash); to be reminded of; to call to mind; to think of; to strike on; to hit on an answer; to seize the crux of a problem
ある理由に思い当たった。
I suddenly realized a reason.
心当たりに思い当たっている。
I am recalling something that comes to mind.
思い当たる原因が多すぎて絞り込めない。
There are too many possible causes to narrow down.