I agree with Blazin, usually I disembark with my primary weapon.
However, last Friday Hot and I played The Wedge in the 1.5 server and every time I disconnected from a boat, helo or motorcycle I was carrying a knife so I'm thinking like many other weird things that happen it's just a glitch in the matrix during any map and not specific to the version.
I've played 1.6 and had it happen both ways. Sometimes I disengage with my primary weapon, sometimes I have my knife.
When I have my knife I simply hit my #3 key and my primary weapon comes up.
I didn't mean to be a smartass in my previous reply Blazin but it does happens so frequently I rarely give it a second thought anymore.