Inconsistent downward velocity with execute, detect, and tp

Using the tp command with execute and detect sometimes increases the downward velocity of the player, sending them hurtling toward the ground. Here is the command used to teleport the player:
/execute @p ~ ~ ~ detect ~ ~-1 ~ minecraft:bedrock -1 tp @p ~ ~20 ~

The purpose of the command is to teleport the player into the air after stepping on the bedrock block (like a jump pad). The effect described above usually occurs after falling off of a ledge onto said bedrock block, without jumping off the ledge. I also run the following commands in the background:
/execute @p ~ ~ ~ testforblock ~ ~-1 ~ minecraft:stained_glass 14
which is meant to kill the player if they step on red stained glass (NOTE: This and the following command do NOT use detect in the command)
/execute @p ~ ~ ~ testforblock ~ ~-1 ~ minecraft:stained_glass 11
which produces the same affect as the first command except the player is teleported after standing on a blue stained glass block, and
/testfor @p[score_SHiFT_min=1]
/tp @p ~ ~-3 ~
/scoreboard players reset @p SHiFT
These commands test if the player is crouching, and if so, teleports them through the ground three blocks. I include these because the teleportation can trigger the bug.