Possible reasons for disconnect reason 0x8

[Sawada]

Central info
   SDK: 15.3
   nRF52840

Peripheral info
   SDK: 13.1
   nRF52832

Number of connected devices
Central 1: Peripheral 15

Connect these devices and leave them alone.
If communication is performed regularly, BLE communication may be unexpectedly disconnected.
(Communication with one of the pheripherals is disconnected.)

Disconnect reason is 0x8 (BLE_HCI_CONNECTION_TIMEOUT).
Why does this disconnect reason occur?

Is it possible to get a list of possible causes?

[smackenzie]

Your connection supervision timeout may be too small.

If no data is received within the supervision timeout, the connection is assumed to be lost. Because you have 15 connections, it could that the softdevice get time to communicate to one of the devices within its timeout.

ble_gap_conn_params_t::conn_sup_timeout

[Sawada]

The following settings are used in all Peripherals.
#define MIN_CONN_INTERVAL MSEC_TO_UNITS (50, UNIT_1_25_MS)
#define MAX_CONN_INTERVAL MSEC_TO_UNITS (65, UNIT_1_25_MS)
#define SLAVE_LATENCY 3
#define CONN_SUP_TIMEOUT MSEC_TO_UNITS (4000, UNIT_10_MS)

The following settings are used in Central.
#define NRF_BLE_SCAN_MIN_CONNECTION_INTERVAL 7.5
#define NRF_BLE_SCAN_MAX_CONNECTION_INTERVAL 30

Isn't this setting enough?