The collateral fee is "charged" when a client misbehaves, i.e. does not comply with the masternode mixing protocol. That is to ensure that (D)DoS attacks against the masternode network are a costly enterprise.

When you connect to a masternode you send it the signed collateral tx but the MN doesn't broadcast it to the network. Think of it as a deposit. If everything goes fine and you comply with the protocol, that signed tx is destroyed, so nothing happens. If your client does things that hurt the normal mode of operation the tx is broadcast to the network and you're "charged" the fee.

For non-malicious nodes this should hardly ever happen once everything is working as intended. Very flaky internet connections might be a problem though.