To give a really complete answer to the question we would need to know a few more things about the situation including whether when the line takes a hit does the line go protocol down or does it stay protocol up but no data passing through. It would also be helpful to know how the line was configured and how EIGRP was configured (in particular are any routers configured to limit EIGRP traffic based on non-default bandwidth restrictions (by default the router will not use more than 50% of configured bandwidth), are any routers configured as stub, are any EIGRP timer configured to non-default values, is any summarization being done).

But not knowing these details there are some things that we can say about differences when you shut the interface vs when the line takes a hit. When you shut the interface it is certain that the router knows that it is losing a neighbor(s) on that interface. Depending on the version of code you are running the router will send an EIGRP "goodby"

out the interface before it shuts the interface down, so that the neighbor knows for sure that the neighbor relationship is being terminated. Both routers must purge from the topology database routes that they had learned from the neighbor, must put into active state and send query messages for any route they were using through the neighbor, must wait for query responses before they can put replacement routes into the routing table. And when you no shut the interface both routers must send and receive EIGRP hello messages, must form neighbor relationships, and must exchange routes from their topology tables before convergence is complete.

Depending on what happens when the line takes a hit some or all of the above steps may not be taken when the line takes a hit. This might account for some of the difference that you are seeing.

HTH

Rick