You are driving along a road, which has an exit to a tunnel which runs fairly close or parallel to the main road.
If the two are close enough together, the GPS may consider that you are still on the main road, and have not taken the exit to the tunnel.
Then, your vehicle enters the tunnel.
Normally, if the GPS knows it is going to enter a tunnel, it will make allowances for the fact that it will lose sight of any satellites until it emerges from the tunnel.
However, if the GPS still thinks you are on the main road, and you "catch it unawares" as you enter the tunnel, the GPS has no way of recognising this, as it has lost sight of any GPS satellites.
The GPS will recalculate once you emerge from the tunnel, but it may sit trying to recalculate during the duration of the tunnel journey.