No surge, but check:

1) how many were from the same (or related) site (or ISP)

2) how many had a similar reason?

3) are there any "hold-ups" on intermediate mail or listserv servers?

4) can you now get to the SMTP port of the deleted subscriber?

5) do you have access to the SMTP delivery (attempt) logs?  was there a
change of IP address of  the target mail exchanger (a frequent problem on
VM/CMS when sending to an ISP that changed IP address within x-days)

/Pete