It is a defence
Many New World tarantulas brush specialised urticating hairs from the abdomen when they feel threatened. The behaviour is intended to create distance.
Common triggers
Opening a retreat, touching the spider, moving the enclosure, rehousing and close maintenance can all provoke hair-kicking.
Back off
If the spider starts kicking, stop escalating the interaction. Give it space and allow it to retreat before continuing non-urgent maintenance.
Protect your eyes
Loose hairs can remain airborne or on webbing and substrate. Keep your face away from the enclosure and wash hands after work.
Species differ
Some species kick readily while others rarely do. Individual spiders also vary. A normally calm animal can still use hairs during stressful maintenance.
Design reduces exposure
Good access, long tools and catch-cup methods reduce the need to put hands close to the spider and therefore reduce hair exposure.