godan verb (-ru), intransitive verb
Intransitive: describes things such as dust, stress, work, or money building up on their own, without an agent. Often used for unwanted buildup.
机の上にほこりがたまっている。
Dust has accumulated on the desk.
ストレスがたまると、体調を崩しやすい。
When stress piles up, you're more likely to get sick.
使わないアプリがスマホにたまってきた。
Unused apps have been piling up on my phone.