As reported in the MM mailing list:
http://lists.freedesktop.org/archives/modemmanager-devel/2015-March/001798.html


Quoting:
I recently ran into an issue with ModemManager (1.4.4) where I created
and connected to a bearer and deleted it without first disconnecting.
I am using a Cinterion PHS8 with two AT ports.  If I delete the bearer
without first disconnecting, the second AT port (on which I was using
pppd) is no longer used by ModemManager as the modem is marked as
forced close.

The issue is easily reproduced by executing the following set of
commands with ModemManager running with a modem with 2 AT ports found
by ModemManager:

    mmcli -m 0 -e
    mmcli -m 0 --create-bearer='apn="myapn"'
    mmcli -b 0 --connect
    mmcli -m 0 --delete-bearer=0
    mmcli -m 0 --create-bearer='apn="myapn"'
    mmcli -b 1 --connect

On connecting to bearer 1, the following error is received:

    Couldn't connect: cannot keep data port open.Could not open \
    serial device ttyUSB3: it has been forced close;

The workaround is pretty straightforward, which is to always
ensure that the bearer is disconnected prior to being removed.  I
figured I would provide this info to the list as this seemed like a
potential bug where MM was not properly cleaning up when removing
the bearer.

The force close appears to occur as a result of
iochannel_input_availble being executed as a result of a hangup
initiated by ModemManager itself:

   ModemManager[3052]: <debug> [1425505651.367703] \
     [mm-port-serial.c:889] common_input_available(): (ttyUSB3) \
     unexpected port hangup!
   ModemManager[3052]: <debug> [1425505651.367854] \
     [mm-port-serial.c:1421] port_serial_close_force(): (ttyUSB3) \
     forced to close port