The association with handling and injection seems very plausible to me.
I don't know how easy it will be to break your rosy of this habit once they slip into it.
Rosies seem to associate(very easily) the opening of their enclosures as time to feed.
Earlier this year I was forced to move my snakes out of my place for ~3 months, and in this time several became very nippy b/c I was unable to handle them in between feedings. I'm still trying to break them of this several months on...
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Thanks,
-JC
rosyboas.to

1.1 San felipe (CB'93)
0.1 ghost San felipe (CB'98)
1.0 ghost San felipe (CB'03)
0.1 het ghost San felipe (CB'04)
1.0 ghost-line San felipe (CB'98)
1.1 hypo Borrego (CB'99)
1.1 albino Whitewater (CB'02)
1.1 double het for snow (CB'95)
1.0 snow rosy (CB'04)
1.1 Joshua Tree (CB'96)
1.0 Bagdad (CB'99)
1.1 Harquahala Mts. (CB'04)
1.1 Temecula albino (CB'02)
0.1 Temecula albino (CB'05)
1.1 Hemet anery (CB'05)
0.1 Boa constrictor (CB'92)