I think it could be due to accumulated stale connections. My suggestions are:

1, Adjusting the maximum pool size would also help.

2, Closing established connections when your app shutsdown so they can be returned back to the pool. Check if you forgot that.

3, As the answer above says, adjusting the settings for connection timeout, and socket timeout and maxidletime might also fix the issue.

You can find the docs here, check it out.