A simple function which uses hardware.wakereason() to reveal why an imp restarted.

wakereason.example.nut

function logWokenReason() {
    local s = "Device restarted. Reason: ";
    local c = hardware.wakereason();
    switch (c) {
        case WAKEREASON_POWER_ON:  // 0
            s += "Cold boot";
            break;
        case WAKEREASON_TIMER:  // 1
            s += "Timer woke the imp";
            break;
        case WAKEREASON_SW_RESET:  // 2
            s += "Software reset or out-of-memory error";
            break;
        case WAKEREASON_PIN:  // 3
            s += "Wakeup pin asserted";
            break;
        case WAKEREASON_NEW_SQUIRREL:  // 4
            s += "Application code updated";
            break;
        case WAKEREASON_SQUIRREL_ERROR:  // 5
            s += "Squirrel error during the last run";
            break;
        case WAKEREASON_NEW_FIRMWARE:  // 6
            s += "Device has a new impOS";
            break;
        case WAKEREASON_SNOOZE:  // 7
            s += "Woken from a snooze-and-retry event";
            break;
        case WAKEREASON_HW_RESET:  // 8
            // Requires imp003 or above
            s += "RESET pin asserted";
            break;
        case WAKEREASON_BLINKUP:  // 9
            s += "Device's network settings have been re-configured";
            break;
        case WAKEREASON_SW_RESTART:  // 10
            s += "Device's application called server.restart()";
            break;
        default:
            s += "Other (code: " + c + ")";
    }

    server.log(s);
}

logWokenReason();