By default, Cisco and Juniper routers (and likely others too, those are just the ones I can speak of) will advertise all bgp learned routes to all bgp peers unless you specify a route map/export policy.

Leaking a route would imply you’re advertising the route to a peer that you’ve otherwise not intended to, either due to a misconfiguration or by not configuring things at all.

most likely these routers in Nigera were configured to send traffic destined to Google through China first ( a more specific /24 out of the Google prefix was configured -> China Telecom ASN ), through a route-map mistake this was advertisied out to the public internet instead of staying within the Nigerian ISP ASN.