Authentication plugins like AuthMe are used to set players movement speed to 0 until the player is logged in. The new collision system in mc 1.9 disallows that two players standing on the same block, so they both knockback each other. I've just tested it on my sandbox server (only Essentials is installed) ..

[16:57:56 INFO]: Testaccount2 issued server command: /tppos 108 68 121
[16:57:58 INFO]: Testaccount2 issued server command: /speed 0
[16:58:10 INFO]: Testaccount issued server command: /speed 0
[16:58:14 INFO]: Testaccount issued server command: /tppos 108 68 121
[16:58:19 WARN]: Testaccount moved too quickly! -3.490137955850514E-4,0.41999998688697815,-2.0755624768753478E-4
[16:58:20 WARN]: Testaccount moved too quickly! 0.0,0.41999998688697815,0.0
[16:58:20 WARN]: Testaccount moved too quickly! 0.0,0.41999998688697815,0.0
[16:58:28 WARN]: Testaccount moved too quickly! -2.8873986593680456E-4,0.41999998688697815,5.5364724175888114E-5
[16:58:35 WARN]: Testaccount moved too quickly! 0.0,0.41999998688697815,0.0