Why are Israeli planes followed by an armored police truck at ZRH?
It is happening as an extra security measure, probably attempting to mitigate the risk of an explosive-laden person or car breaching the perimeter fence - like many city airports ZRH has a multitude of roads running around the perimeter fence which cannot all be manned 24/7. Is it mostly for show? Who knows!
There has always been more risk associated with flying to Israel - anyone who flew there pre-9/11 was not surprised by the extra security measures added to all flights post-9/11 as they had been dealing with them since forever.
It seems that American might also get the same treatment at Zurich based on a photo here: https://www.airliners.net/photo/American-Airlines/Boeing-767-323-ER/531127/L